Hello;
Using Firebird 5.0.3 64 bit I found this case today (same in 3 diferent database and machine).
It seems the slave-side received a segment (corrupt, I guess ...), and Firebird is not accepting it. The point is that Firebird is not using it for updating the database, but it does not report any issue on the replication.log file.
When I overrided this segment with a new copy (not corrupted, I guess) then Firebird used it.
I attached here the control file and the segment, maybe this is a minor bug that may be fixed.
Regards.