ORA-00940: invalid ALTER command
I eventually managed to narrow this down to a “_fix_control” setting.
The database is for an SAP system and the init.ora was updated after following SAP note 830576 (Parameter recommendations for Oracle 10g).
I noticed that I had misread the note and applied a “_fix_control” parameter setting for a slightly (only by a patchset update) higher version of Oracle.
Take care when reading this note and make sure you test the startup and shutdown of the database before going home!
UPDATE: This is a bug. See here.