And from the release notes of FB 1.5 I found this:

Services API is now supported on Classic
(Nickolay Samofatov)
Features of GSEC and GSTAT are now supported via the Services API in
CS builds. It means that the
entire Services API now works in both Superserver and Classic
architectures.
Known issue: GSEC doesn't deliver error status vectors to the client
side if forked from the CS process.
Any error will prevent the security database from being changed; but
the exception is decoded by the
server and the appropriate message is delivered via the API
communication buffer instead of the status
vector. If this situation is not handled properly by the user program,
it may cause application-specific
errors. For example, IBExpert displays the message "unexpected output
buffer value".


Doesn't it sound like this error ?
Not that I know how to solve it ...