Hi Jörg,
Did this work before and the problem just started? Did you make a change is now running on 64-bit IIS just like GeishaMX? There was a problem discussed in this thread:
Dynamic Forms - 64 Bit Operating System Issues.
If this is the same situation, I would recommend an upgrade to
Dynamic Forms 3.4 in order to get the latest fixes and enhancements. Thanks!
Candace