I'm currently experiencing this. I have a days worth of unsaved work open on Construct 2. It does not save even though I make a change and it gives a progress bar, but the date and time don't change on the .caproj file. It's a fairly large project also. I've been working on my game for half a year now. I've experienced this before and I lost a week worth of work. Last time I thought it was hardware failure, but I've since switched computers and now it happened again.
In short: Construct2 looks like it's saving, but there's no changes to the project files. I also get these capx.capx.capx files.
Possible cause:
My computer was resuming from sleep when the "autosave" popped up on construct.
Any interruption to "autosave" corrupts the project and it is unable to save from this point on.