>> Does anybody have a clue what's going wrong here?

> He uses ordinary client library instead of embedded engine.

I can exclude that. As you can see in my list, fbembed.dll is in the
application directory. My program finds this and uses it as the client
library.

Even on my development machine where there are three fully installed
Firebirds (1.5, 2.0, 2.1) and one of them (2.1) is registered via
instreg, the program finds and uses the local fbembed.dll

Best Regards

Stefan