I've seen similar error messages before this version of C3, then the project just won't open anymore. I even extracted my C3P to look at the folders/json and everything looked intact at a glance. I had assumed this was just me breaking something with some 3rd party plugins I'd been trying out, but I saw a couple other people in this forum report similar issues. For example, the litetween thread in the C2 section has a report from someone whose C3 project won't open now - same thing. Previously-working project failing to load with a "<whatever> is not a function" error message.
As for this version of C3, I haven't tried Dropbox, but I've been having no luck getting OneDrive to open through the Cloud file open dialog. I get a Microsoft page saying there are problems with the service and to try back later, but I'm able to access my files just fine by browsing to them with the OneDrive web interface outside of C3. I have the previous version of C3 still installed on another machine, thankfully, and it is opening those files just fine.
Sure would be nice if there was some easy way to roll back C3 to a previous version, but I'm not aware of any.