Yep, $SYNC will not help here.
If the buffer is corrupt, then in SU53 via the menu you can reset the buffer. This is the same as report RSUSR405 or SU01 command RSET which sets the flag to rebuild the buffer when you next logon. So just logoff like in the good old days of 46B.
If the PFCG generation is corrupt, then start tcode SUIM_OLD. This will automatically run FM SUSR_SYNC_USER_TABLES and sort out orphaned authorizations and also inconsistencies between the UST and USR tables. Generate the profiles in PFCG afterwards to be on the safe side.
But SVEN is correct. Without providing information about OOAC, this is all guessing as that should come first in the troubleshooting box of tricks.
Cheers,
Julius