XPRESS Licensing Error (Error nv01) on Server Environment – Single Use Class 2

[StatNI]

Hello everyone,
we recently received the XPRESS Solver and set it up following FICO's instructions. After some initial unsuccessful attempts, we were at least able to run two error-free instances using version 3.5.0 Build 27. Higher versions crashed immediately with an error message. In the meantime, we are now also receiving an error in version 3.5 – despite using the same data and inputs as in the two successful runs.
The error messages in detail:

3.5.0:
Xpress licensing error 9: The host ID of this machine could not be detected. Please contact the Xpress support team. (Error nv01)

4.2.5 Build 2:
JJ-routines: XPress license error
libTauHiTaS version is 4.3.2.1
Using XPRESS
Exited with code 8000: JJ-routines: XPress license error

Regarding our setup: We are running the solver in a server environment (no local installation), configured as Single Use – Class 2 for multiple users. All errors occur on the same server. Nothing has changed in the configuration between the successful and the failed runs.
Particularly notable regarding error nv01: The host ID reported by the system is identical to the host ID we submitted to FICO during licensing – so there does not appear to be a discrepancy at that level.

Troubleshooting steps we have already taken:

• Tested with input data from a colleague's office that had previously run successfully with XPRESS – also without success

• Replaced the DLL files xprl.dll and xprs.dll as described in several forum posts – unfortunately also without success

What puzzles us most is that version 3.5 worked temporarily, without any apparent explanation for the subsequent failure.
Does anyone have an idea or a possible solution?

Thank you very much!