luptak, have you explored using the command line tool /opt/qradar/support/qappmanager?
you should be able to use this tool to remove both of the failed UBA instances and app definintions.
Here are the two options you want to explore:
10) App definition - list all
14) App definition - delete
20) App instance - list all
29) App instance - delete
Basically use the list all options to find the app id of the UBA instances and definitions, then use the delete options to remove them. This tool will show items not seen in the Extensions Manager in QRadar Console UI.
I hope this helps!
#QRadar#Support#SupportMigration