blank window with XDMCP between linux and unix

I have a Digital Unix 4.0 emulation running without a display (FreeAXP). I have an X app (Open Genera 2) on this host that I would like to use. I managed to configure both Unix and Linux and I can do X login from Linux (Debian 11 at the moment) to Unix. Both X and CDE environments seem to be working fine. However, when I run the mentioned application, I only see a (white) blank window without any contents. I tried a different X server (on Windows) and also not using XDM but setting display explicitly and running the program from telnet, but no change. I can see the network traffic (on 6000/tcp) with tcpdump on Unix. I am considering using Xvfb on Unix to see if it works locally and also proxy/debug X messages on Linux, but I dont see why it would be different. What am I missing ?