You are a genius @diesieben07! it worked! thank you very much for your help.
For context, this IS running on a headless Linux Mint machine, but had not thought about monitor issues as other games run ok and I would swear I had previous versions of Forge running in this same setup, albeit that was long long ago. I guess I´ll be buying one of those dummy HMDI connectors.
What line and log did you detect the issue at?.
Let me know if I can provide you with any logs or details if you do end up raising that patch request.