Internal gds software consistency check. Pointer page vanished from mark_full
You probably reached a limit that restricts the record space for a table to about 70 million kB, including record headers and back versions (not counting large BLOBs).
A backup/restore cycle will fix the problem temporarily, as is clears out the old record versions. To fix it in the long run, you should either move some of the records to another table or use Firebird 2 if you are still using Firebird 1.x.