Comment # 10 on bug 1216969 from Michael Hirmke

(In reply to Fabian Vogt from comment #9)
> Sounds like .local/share/kactivitymanagerd/resources/database is corrupt.
> 
> You can log out and delete it (make a backup if necessary), this will
> trigger recreation of a new empty database on the next login.

I logged out, renamed the directory and logged in again.
New database files are created, but the problem persists - no recent files or
apps.
There is an error.log in this directoy containing one line:

024-05-22T00:00:32 error: near ".": syntax error Unable to execute statement