Autosaving, manual saving, update installation and probably many more features simply do not work with “Operation failed” being captioned underneath all of them.
I tried reinstalling, updating and changing directories - no dice.
Only thing that seems to work is Image Alignment and Normal Reconstruction after pressing Start, although the process crashes shortly after. I have no idea what may be causing such issue and how to resolve it.
Well that was dumb. Solution was to log in using Epic Games account, which I had to create.