I opened a project this morning in the r83 beta that I had created with the previous release (r82), made various changes and additions over the course of about an hour, testing and saving several times. Then, trying one more change and save, I got this message: "Failed to move uistate.xml.tmp to uistate.xml!"
Repeated attempts to save or save-as just prompted the same message to appear, and no save was ever completed. I finally closed Construct 2.
Even worse, not only was no save ever completed, but the file I had opened to start with had also disappeared! In other words, even the most recent successful save was gone. (It was the single-file type of project.) Luckily, I had a backup stored elsewhere (although that file does not reflect the state the project was in at the time this error happened, so I see no reason to attach it).
I understand r83 is a beta release and was just playing around with it. But I'm going to download and use the last stable version.