No luck still 
If it helps any: here’s the strace output for an update attempt using the default delta update build (without the -n for xdelta3).
$ ls -l A2B.mender
-rw-rw-r-- 1 kris kris 230K Mar 18 15:18 A2B.mender
$ sudo strace mender install A2B.mender
...
futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULLINFO[0000] Mender running on partition: /dev/mmcblk0p1
INFO[0000] Start updating from local image file: [A2B.mender]
Installing Artifact of size 242176...
INFO[0000] No public key was provided for authenticating the artifact
- 1 %) = 0
sched_yield() = 0
futex(0xb81838, FUTEX_WAKE_PRIVATE, 1) = 0
futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULL) = 0
futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULL) = 0
nanosleep({tv_sec=0, tv_nsec=3000}, NULL) = 0
futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULL) = 0
futex(0x400003f648, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0x400003f648, FUTEX_WAKE_PRIVATE, 1) = 1
. - 2 %) = 0
futex(0x4000080848, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0x4000080848, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0xb81d88, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0x400003f648, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0xb81950, FUTEX_WAIT_PRIVATE, 0, {tv_sec=0, tv_nsec=100000}) = -1 EAGAIN (Resource temporarily unavailable)
epoll_pwait(4, [], 128, 0, NULL, 12065536) = 0
futex(0x400003f648, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0xb81950, FUTEX_WAIT_PRIVATE, 0, {tv_sec=0, tv_nsec=100000}) = -1 ETIMEDOUT (Connection timed out)
.. - 4 %) = 0
.............................................................. - 100 %futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULL
) = 0
futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULL) = 0
futex(0xb823e8, FUTEX_WAIT_PRIVATE, 0, NULL) = 0
futex(0x4000200148, FUTEX_WAKE_PRIVATE, 1) = 1
read(12, 0x400019a000, 32768) = -1 EAGAIN (Resource temporarily unavailable)
read(14, 0x40001a2000, 32768) = -1 EAGAIN (Resource temporarily unavailable)
waitid(P_PID, 12046, INFO[0000] Update module output: xdelta3: target window checksum mismatch: XD3_INVALID_INPUT
INFO[0000] Update module output: xdelta3: normally this indicates that the source file is incorrect
INFO[0000] Update module output: xdelta3: please verify the source file with sha1sum or equivalent
INFO[0000] Update module output: Failed to apply the delta, err: 1
{si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12046, si_uid=0, si_status=1, si_utime=0, si_stime=0}, WEXITED|WNOWAIT, NULL) = 0
wait4(12046, [{WIFEXITED(s) && WEXITSTATUS(s) == 1}], 0, {ru_utime={tv_sec=0, tv_usec=40000}, ru_stime={tv_sec=0, tv_usec=52000}, ...}) = 12046
futex(0x40002004c8, FUTEX_WAKE_PRIVATE, 1) = 1
openat(AT_FDCWD, "/var/lib/mender/modules/v3/payloads/0000/tree/stream-next", O_RDONLY|O_NONBLOCK|O_CLOEXEC) = 12
epoll_ctl(4, EPOLL_CTL_ADD, 12, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158537848, u64=547619384440}}) = 0
fcntl(12, F_GETFL) = 0x20800 (flags O_RDONLY|O_NONBLOCK|O_LARGEFILE)
fcntl(12, F_SETFL, O_RDONLY|O_NONBLOCK|O_LARGEFILE) = 0
epoll_ctl(4, EPOLL_CTL_DEL, 12, 0x40000cdc88) = 0
close(12) = 0
futex(0x4000200148, FUTEX_WAKE_PRIVATE, 1) = 1
openat(AT_FDCWD, "/etc//localtime", O_RDONLY) = 11
read(11, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 111
read(11, "", 4096) = 0
close(11) = 0
getpid() = 12025
write(3, "<11>Mar 18 14:15:19 mender[12025"..., 124) = 124
write(2, "\33[31mERRO\33[0m[0000] Download fai"..., 89ERRO[0000] Download failed: Update module terminated abnormally: exit status 1
) = 89
openat(AT_FDCWD, "/etc/mender/scripts", O_RDONLY|O_CLOEXEC) = 11
epoll_ctl(4, EPOLL_CTL_ADD, 11, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158537624, u64=547619384216}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 11, 0x4000184b38) = -1 EPERM (Operation not permitted)
getdents64(11, /* 3 entries */, 8192) = 80
getdents64(11, /* 0 entries */, 8192) = 0
newfstatat(AT_FDCWD, "/etc/mender/scripts/version", {st_mode=S_IFREG|0644, st_size=1, ...}, AT_SYMLINK_NOFOLLOW) = 0
close(11) = 0
openat(AT_FDCWD, "/etc/mender/scripts/version", O_RDONLY|O_CLOEXEC) = 11
epoll_ctl(4, EPOLL_CTL_ADD, 11, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158537624, u64=547619384216}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 11, 0x4000184bd8) = -1 EPERM (Operation not permitted)
read(11, "3", 512) = 1
read(11, "", 1535) = 0
newfstatat(AT_FDCWD, "/var/lib/mender/modules/v3/payloads/0000/tree", {st_mode=S_IFDIR|0700, st_size=1024, ...}, 0) = 0
openat(AT_FDCWD, "/dev/null", O_RDONLY|O_CLOEXEC) = 12
epoll_ctl(4, EPOLL_CTL_ADD, 12, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158537624, u64=547619384216}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 12, 0x4000184ba8) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 12, 0x4000184ba8) = -1 EPERM (Operation not permitted)
pipe2([13, 14], O_CLOEXEC) = 0
epoll_ctl(4, EPOLL_CTL_ADD, 13, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158537624, u64=547619384216}}) = 0
fcntl(13, F_GETFL) = 0 (flags O_RDONLY)
fcntl(13, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
epoll_ctl(4, EPOLL_CTL_ADD, 14, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158537848, u64=547619384440}}) = 0
fcntl(14, F_GETFL) = 0x1 (flags O_WRONLY)
fcntl(14, F_SETFL, O_WRONLY|O_NONBLOCK) = 0
pipe2([15, 16], O_CLOEXEC) = 0
epoll_ctl(4, EPOLL_CTL_ADD, 15, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538072, u64=547619384664}}) = 0
fcntl(15, F_GETFL) = 0 (flags O_RDONLY)
fcntl(15, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
epoll_ctl(4, EPOLL_CTL_ADD, 16, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538296, u64=547619384888}}) = 0
fcntl(16, F_GETFL) = 0x1 (flags O_WRONLY)
fcntl(16, F_SETFL, O_WRONLY|O_NONBLOCK) = 0
fcntl(14, F_GETFL) = 0x801 (flags O_WRONLY|O_NONBLOCK)
fcntl(14, F_SETFL, O_WRONLY) = 0
fcntl(16, F_GETFL) = 0x801 (flags O_WRONLY|O_NONBLOCK)
fcntl(16, F_SETFL, O_WRONLY) = 0
pipe2([17, 18], O_CLOEXEC) = 0
getpid() = 12025
rt_sigprocmask(SIG_SETMASK, NULL, [], 8) = 0
rt_sigprocmask(SIG_SETMASK, ~[], NULL, 8) = 0
clone(child_stack=NULL, flags=CLONE_VM|CLONE_VFORK|SIGCHLD) = 12056
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
close(18) = 0
read(17, "", 8) = 0
close(17) = 0
close(12) = 0
epoll_ctl(4, EPOLL_CTL_DEL, 14, 0x4000184c48) = 0
close(14) = 0
epoll_ctl(4, EPOLL_CTL_DEL, 16, 0x4000184c48) = 0
close(16) = 0
futex(0x4000200148, FUTEX_WAKE_PRIVATE, 1) = 1
write(6, "\0", 1) = 1
waitid(P_PID, 12056, {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12056, si_uid=0, si_status=0, si_utime=0, si_stime=0}, WEXITED|WNOWAIT, NULL) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12056, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
rt_sigreturn({mask=[]}) = 0
futex(0xb81938, FUTEX_WAKE_PRIVATE, 1) = 1
wait4(12056, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, {ru_utime={tv_sec=0, tv_usec=4000}, ru_stime={tv_sec=0, tv_usec=0}, ...}) = 12056
unlinkat(AT_FDCWD, "/var/lib/mender/modules/v3/payloads/0000/tree", 0) = -1 EISDIR (Is a directory)
unlinkat(AT_FDCWD, "/var/lib/mender/modules/v3/payloads/0000/tree", AT_REMOVEDIR) = -1 ENOTEMPTY (Directory not empty)
openat(AT_FDCWD, "/var/lib/mender/modules/v3/payloads/0000", O_RDONLY|O_CLOEXEC) = 12
epoll_ctl(4, EPOLL_CTL_ADD, 12, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538072, u64=547619384664}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 12, 0x4000184dd8) = -1 EPERM (Operation not permitted)
unlinkat(12, "tree", 0) = -1 EISDIR (Is a directory)
newfstatat(12, "tree", {st_mode=S_IFDIR|0700, st_size=1024, ...}, AT_SYMLINK_NOFOLLOW) = 0
openat(12, "tree", O_RDONLY|O_CLOEXEC) = 13
epoll_ctl(4, EPOLL_CTL_ADD, 13, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538072, u64=547619384664}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 13, 0x4000184c98) = -1 EPERM (Operation not permitted)
getdents64(13, /* 10 entries */, 8192) = 336
getdents64(13, /* 0 entries */, 8192) = 0
unlinkat(13, "current_device_type", 0) = 0
unlinkat(13, "header", 0) = -1 EISDIR (Is a directory)
newfstatat(13, "header", {st_mode=S_IFDIR|0700, st_size=1024, ...}, AT_SYMLINK_NOFOLLOW) = 0
openat(13, "header", O_RDONLY|O_CLOEXEC) = 14
epoll_ctl(4, EPOLL_CTL_ADD, 14, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538072, u64=547619384664}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 14, 0x4000184af8) = -1 EPERM (Operation not permitted)
getdents64(14, /* 8 entries */, 8192) = 256
getdents64(14, /* 0 entries */, 8192) = 0
unlinkat(14, "artifact_group", 0) = 0
unlinkat(14, "artifact_name", 0) = 0
unlinkat(14, "type-info", 0) = 0
unlinkat(14, "header-info", 0) = 0
unlinkat(14, "meta-data", 0) = 0
unlinkat(14, "payload_type", 0) = 0
close(14) = 0
unlinkat(13, "header", AT_REMOVEDIR) = 0
unlinkat(13, "version", 0) = 0
unlinkat(13, "tmp", 0) = -1 EISDIR (Is a directory)
newfstatat(13, "tmp", {st_mode=S_IFDIR|0700, st_size=1024, ...}, AT_SYMLINK_NOFOLLOW) = 0
openat(13, "tmp", O_RDONLY|O_CLOEXEC) = 14
epoll_ctl(4, EPOLL_CTL_ADD, 14, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538072, u64=547619384664}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 14, 0x4000184af8) = -1 EPERM (Operation not permitted)
getdents64(14, /* 2 entries */, 8192) = 48
getdents64(14, /* 0 entries */, 8192) = 0
close(14) = 0
unlinkat(13, "tmp", AT_REMOVEDIR) = 0
unlinkat(13, "stream-next", 0) = 0
unlinkat(13, "streams", 0) = -1 EISDIR (Is a directory)
newfstatat(13, "streams", {st_mode=S_IFDIR|0700, st_size=1024, ...}, AT_SYMLINK_NOFOLLOW) = 0
openat(13, "streams", O_RDONLY|O_CLOEXEC) = 14
epoll_ctl(4, EPOLL_CTL_ADD, 14, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=2158538072, u64=547619384664}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 14, 0x4000184af8) = -1 EPERM (Operation not permitted)
getdents64(14, /* 3 entries */, 8192) = 120
getdents64(14, /* 0 entries */, 8192) = 0
unlinkat(14, "A.ext4.delta", 0) = 0
close(14) = 0
unlinkat(13, "streams", AT_REMOVEDIR) = 0
unlinkat(13, "current_artifact_name", 0) = 0
unlinkat(13, "current_artifact_group", 0) = 0
close(13) = 0
unlinkat(12, "tree", AT_REMOVEDIR) = 0
close(12) = 0
close(10) = 0
futex(0x400003f648, FUTEX_WAKE_PRIVATE, 1) = 1
ERRO[0000] Update module terminated abnormally: exit status 1
+++ exited with 1 +++
Does anyone else perhaps have an idea?
B, and 
B. If I try to install the B2A variant, I actually get the expected error with a checksum failure: