This was brought up when AC27 was first released. There is a thread around here somewhere...
Probably in the beta or tech preview forums which are closed now.
I can't recall the solution though.
I seem to recall there being posts about it, but maybe I am just imagining that.
I can't remember if it was in the Beta testing or the Tech Preview.
It seems I can still access the Beta bug reports, but I couldn't see anything there.
I can't access the Tech Preview forum.
And I can't remember if there was a solution or not.
I think I even experienced it myself in the first Beta version.
I don't remember doing anything to fix it other than version updates.
Sorry I can't help more.
Sometimes just closing all running Archicad's helps.
As next step, try deleting %USERPROFILE%\AppData\Local\Graphisoft\Monaco-... before starting Archicad.
If these don't solve it, the problem might be that your Windows username contains non-ASCII characters. It will be fixed in the next update.
I think Peter has the solution.
I can remember that being discussed before now.