Hi helloimneil, (OR do you prefer Neil?),
I checked my version here, (and I’m not aware which one you use) and I have no problems.
Sudden stops, with no other changes, would lead me to ask to open a new scene file and check there, restart eventually. But that is wild guessing and perhaps only waste time on your side.
Since it came suddenly, I hesitate to suggest the Support route, but having nothing else to go on, here we go:
https://www.maxon.net/en-us/support/how-can-we-help/
Of course, I’m happy to look into it as well, if you like an upload link, to keep the file private, let me know.
Edit/Added:
Have you tried to work with CMD/CTRL-C and then continue always with CMD/CTLR V?
I tried to reproduce it with the Preference> Input Device Setting> Control Click for Right-Click.
/edit
Cheers