> So, where to look for?
> Why suddenly some users report there is no read/access to RDB$RELATIONS and
> another system tables?

I have no idea, but you can try these steps:

1) Upgrade server to 2.1.4
2) Recreate database from script (not just backup-restore)

If the problem persist,

3) check if the query fails after metadata-only backup-restore

and if it does,

4) make this metadata-only backup and the query available to FB developers.

SY, SD.