hey @Piocky
thanks for using Mender. perhaps we should start a new topic, since your issue is a bit different.
what I would suggest is: backup the inventory devices collection, then clean it:
use inventory...
db.devices.deleteMany({})
after that, I would suggest to run the deviceauth commands to propagate the data to inventory:
docker exec -i `docker ps | grep device-auth | head -1 | awk '{print($1);}'` deviceauth propagate-inventory-statuses
docker exec -i `docker ps | grep device-auth | head -1 | awk '{print($1);}'` deviceauth propagate-inventory-id-data
it is a bit late on Friday night, so just double check that those commands exist in 3.1. on that note using the Mender Server 3.1 does not really make much sense, you should upgrade to 3.6 as soon as you can.
one more thing: the serial number as such is not necessarily a problem, as long as the whole identity is unique (I hope you have something more there than just the serial).
best regards,
peter