Thank you very much for your many efforts. Now I am more than tempted to check out Endeavor OS, because the desktop build refused to start at all... Seems to be an issue with the Manjaro flavored mono build (unfortunately not the first time I encounter this)... It still could be the old hardware though; but if Manjaro refuses to open it on my Threadripper machine...
For completeness (I am no C#/Mono Expert), here is the error log:
Unhandled Exception:
System.TypeInitializationException: The type initializer for 'System.Random' threw an exception. ---> System.TypeInitializationException: The type initializer for 'Sys' threw an exception. ---> System.DllNotFoundException: System.Native assembly:<unknown assembly> type:<unknown type> member:(null)
at (wrapper managed-to-native) Interop+Sys.LChflagsCanSetHiddenFlag()
at Interop+Sys..cctor () [0x00000] in <02071a39c8a34187bdd1f58e4be38764>:0 --- End of inner exception stack trace ---
at Interop.GetRandomBytes (System.Byte* buffer, System.Int32 length) [0x00000] in <02071a39c8a34187bdd1f58e4be38764>:0 at System.Random.GenerateGlobalSeed () [0x00000] in <02071a39c8a34187bdd1f58e4be38764>:0 at System.Random..cctor () [0x00000] in <02071a39c8a34187bdd1f58e4be38764>:0 --- End of inner exception stack trace ---
at SpaceExplorer.Scripts.Actor.Player..ctor () [0x00036] in <b6760ce72c3f4c88bbc4c0268582e59c>:0