basicUserRegistry.xml
Accepted answer
The basic user registry does not erase itself. If it is used, It should also be part of the backup procedure.
If you do not have a backup, you can try to follow the documentation https://www.ibm.com/docs/en/was-liberty/base?topic=liberty-configuring-basic-user-registry and try to recreate the XML file. If that does not work, you will either have to restore a backup of the file, or reinstall.