I quick follow up.
Ashley quickly checked my .capx and found the causes:
1) I had a trailing space in my event sheet name. I.e. "EventsMenu " instead of "EventsMenu". This should be fixed in the next C3 release.
2) One of my variables (a local) had the same name ("sign") as a new system variable. Easy to correct but let's hope the error message will specify name conflicts in the future.
After my corrections I could import my .capx without any problem. LiteTween was my only 3rd part add-on and as I had installed the C3 version of LiteTween, all my LiteTween behaviours were also imported (but don't work with the C3 runtime).