Profibus Prm Fault

We manufacture a Profibus adapter based on the Siemens SPC3 ASIC. This has been in production for something like fifteen years now - it is stable and I know it works.

We recently got an HMS PDP-SIM, but when connected to our device, the Master Simulator tells me:
Communication Active - The Diagnostic telegram is working fine, our fault codes are being passed through as expected, and the Ext Dia indicator on the simulator responds as expected.

Status shows as “Not connected”, and our device also indicates that it is not in Data Exchange state (the SPC3 turns a LED on when this is achieved).

The simulator is also indicates that there is a “Prm Fault”.
An edited version of GSD file has no ExtUserPrmData references, and passes the test in the the latest version (V6.0.0) of the official Profibus GSD Editor with no errors or warnings, so I have no reason to believe the problem is there.
Any ideas about what causes the Prm fault?
Any ideas about the connection

I am not sure about this one. It may be that the parameter configuration configured on the device do not match how the master is configured.