I had some trouble with FB last night - maybe it interest someone:

I had IB6 on my system (WinXP Home) running as service
I stopped the server (didn't uninstall it).
Then installed FB ver WI-V1.0.0.794 running as service

When I tried to connect to database with charset other than NONE, I got error:
ISC ERROR MESSAGE:
Dynamic SQL Error
SQL error code = -204
Table unknown
RDB$PROCEDURES

When I set charset to NONE - it worked.
When I unchecked "Run Interbase server as a service" it also started to work.

So finaly I uninstalled IB and reinstlled FB, then everything worked OK.

Maybe it helps someone who switch databases

Riho-Rene Ellermaa
senior programmer
Hansabank