F:\fb3dev\fbbuild\firebird30\temp\Win32\Debug\firebird\bin>isql
Use CONNECT or CREATE DATABASE to specify a database
SQL> create database 'sharks';
SQL> alter database add difference file 'tuna';
SQL> alter database begin backup;
SQL> alter database end backup;
SQL> alter database drop difference file;
SQL> alter database begin backup;
Statement failed, SQLSTATE = 42000
unsuccessful metadata update
-STORE RDB$FILES failed
-message length error (encountered 278, expected 276)
SQL>

This happens because drq_s_difference has been reused in dyn_mod (it was used in dyn_def) with an extra null flag being referenced. This causes the total length of the message to differ in two bytes (the null flag is USHORT) and fail. The solution is to assign a new drq.