Just upgraded to PC version 3.1.0; set up a flight EGTF to EDLA (Arnsberg/Menden), and tried to save it but the programme would pop up a crash error window (somewhere in the debug it says "flightplan name not valid").  Airport names without a "/" like in EDLA seem to be ok.  How to circumvent, as you can't even get to the window where you could rename the flightplan?
Strangely enough saving to cloud works.

Slashes are not valid in a filename.

I can't replicate your problem though in the sense that when I press save, I can change the name of the file before pressing the save button.

This only happens on Windows XP, which is no longer supported by Microsoft. After being alerted to this fact we have changed SkyDemon so that it removes any slashes from the proposed filename, so in the forthcoming version you won't be prevented by Windows from seeing the Save dialog.
In the meantime, the cloud storage workaround is a good one.