> jdbc:firebirdsql:localhost:c:/wie über dieses/wie über dieses.fdb

Can you try

jdbc:firebirdsql:native:localhost:c:/wie über dieses/wie über dieses.fdb

This will use the JNI library as in embedded mode, and if it does not work
we will know that the error is in Jaybird. Otheriwse the issue is with
embedded engine and we will need to contact either firebird-devel or
firebird-support.

Roman