Firs of all I want to thank all people who took the time and tried to
help me. I really appreciate it!

My connection details:
LibraryName=fbclient.dll
DBName=localhost/3121:C:\xxx.FDB
CharSet=ISO8859_1
SQLDialect=3

I start the Firebird 1.5 Server with "fbserver -a -i -p 3121"

Your remarks helped me to make a few more tests. Here are the results:
I installed IBExpert and tried to connect to it and it failed as
well. The error message was:

"This user does not have privilege to perform this operation on this
object. no permission for read-write access to database C:\xxx.FDB"

Then I tried to connect with IBExpert to a different database and it
worked fine. Then I let my program connect to the different database
and it worked also!!

So, is it a problem of the two FDB files? There are no differences in
the file properties (apart from size and path).

Then I copied the C:\xxx.FDB (which wasn't working) to the same
directory where the other database file was. And then my program
could connect to it.

Can it be a problem of the directory?

Regards,

Johanna.