Subject nbackup problem : Database is already in the physical backup mode

We have a POS application using Firebird 2.5.3 together with JayBird on about 300 sites.

The applications runs nbackup several times everyday. It has happened several times on several sites that nbackup fails with following exception:
org.firebirdsql.jdbc.FBSQLException: GDS Exception. 335544351. unsuccessful metadata update
Database is already in the physical backup mode

What happes after this is that all database changes from that point get stored in the .delta file.

The soulution i found is to make a full gbak and restore it.

The is serious nbackup issue which causes problem for the database in general.
The only way to discover this problem is to look after a .delta file or attempt another run nbackup again.
I dont think it's  database corruption or disk problem because it has happen too often and everything works fine after the restoring from gbak.

I have a copy of such corrupted database if it's interesting.