Hi Karol,
thank you for your advice. But during the clearing of 64 Bit versions of the cleint DLLs I also made sure only fbclient.dlls are present.

But I found the solution to the problem and it is only related with our application and I must say it is very basic. The application tries to connect with a specific user to the firebird DB and when this user is not present in the repective security2.fdb it closes with a message "cannot connect". After adding the user to the security fdb everything works fine..
Again thank you very much and please excuse the profanity of the problem . . .  ;-)
Kind regards from the port of Hamburg Germany
Christian