Because of the new behaviour of Firebird 3.0.X regarding explicit nested inner joins (where in the "on" part, a reference to a table column on a higher level in the hierarchy is not allowed anymore), I want to switch back to Firebird 2.5.x. I made a transportable backup from the 3.0 database, but got an error restoring this backup with Firebird 2.5.x:

Connecting to server localhost...
Database restore started 17:23:28
gbak:opened file D:\FIREBIRD_3_0.fbk
gbak:transportable backup -- data in XDR format
Database restore canceled 17:23:28 due to IBPP exception:

*** IBPP::SQLException ***
Context: ServiceImpl::Wait
Message: isc_service_query failed

SQL Message : -901
Unsuccessful execution caused by system error that does not preclude successful execution of subsequent statements

Engine Code    : 336330796
Engine Message :
Expected backup version 1..9.  Found 10
Exiting before completion due to errors

Backup was made and restored with FlameRobin.