When Mender enabled devices have their certificate regenerated, then it is a bit unclear how to use them properly in the Mender web UI. This will happen frequently during development as the devices are reimaged and have a completely new image installed which will effectively, unless you take other precautions, delete and recreate the certificate.
In these situations, if the device is already authenticated to a Mender server, then there will be a new authentication set associated with the device that needs to be accepted. This will be shown in the devices tab as shown below:
When you expand the device details, there is a notification that “This device has a pending authentication set.”:
If you then click the warning text, you will open up the Authorization Status dialog where you will see the pending authentication set:
Simply accept the latest inactive authentication set and your device will be readmitted to the fleet.
Alternatively, you can simply click the red DECOMMISSION DEVICE text to have the device removed completely from your fleet. On the next polling interval the device will be available in pending state.