Unsuccessful metadata update, TABLE xyz, too many versions
Each change you make in table's structure is recorded in RDB$FORMATS system table. When you make 255 changes, you must do a backup and subsequent restore - which resets counter for all tables.
Why is this recording of changes needed? Well, imagine you have a table column, type INTEGER, and you change it to DECIMAL(16,2). When this happens, you may have thousands or millions of records in that table. Internal representation (binary) of these two types is not the same - so it would mean that Firebird needs to rewrite all those records. Instead of doing that, Firebird remembers that those old records have INTEGER format and converts them on-the-fly when reading. New records get stored in DECIMAL(16,2) format.