In 1.3 and newer we do not allow to create users that already exist in the system and have UID < 1000 (https://github.com/vyos/vyos-1x/blob/e29c53bc1d66ed50aaca6f1fc39888c6d5779c9b/src/conf_mode/system-login.py#L106).
However, this was possible in 1.2 and such users migrate to the new config.boot during an update.
During boot, such config.boot file cannot be applied and none of the users are added to the system. So, it is not possible to log in.
We need to fix the migration to avoid this situation.