Similar thing happened a while back to me, now it happened with browser version ( and yeah, just auto-updated even though I said not now ).
Short answer, you have to use browser version until ( if ) this gets resolved.
I was never able to use desktop version again.
I cannot open any of my Construct 2 files in Construct 3 now
I can open them if using Firefox, but then I get failed to open preview error.
This has been going on for me since Chrome version included save to disk feature by default.
I can open files in version 210, before this "feature" was added
I still develop in Construct 2, then use Construct 3 for exports and right now only r210 works ( browser )