Tried to search on http://127.0.0.1:8888/lucee/admin/server.cfm?action=ext.applications via the search form in the Not installed section, threw "variable [DATA] doesn't exist" error.
Now even visiting that section in the admin throws this same error.
Debian, default Lucee install
The issue was fixed 5.1.x version onwards. So, I closed this ticket.