Hi again,
due to your helpful proposals I installed a firebird client directly on the server and integrated an 'old' Windows7 computer into the domain and, surprise surprise, both systems run for several days without a program crash, which leads me to conclude, that neither my program nor the firebird database is the crash source.
Now is time for the IT company to act, because they always rejected, that their new installed system is responsible for the problems.
Thank you guys for your support, I have learned a lot.