okay….after now checking this multiple times, I’ve found what seems to be causing this. When the scaling in Windows10 (–> Einstellungen –> Bildschirm –> Größe von Text, Apps und anderen Elementen ändern: XXX%) is NOT set at 100%, the mouse in TF is limited (no matter if dpi-scaling for the trainfever.exe is enabled or not). Once I set the windows scaling to 100%, everything works fine (except that everything in windows looks too tiny for my tired Feierabend-eyes to see).
Please, please, fix this in the next update as changing back and forth the scaling just to have a functional train fever is significantly killing the fun.