I assume the autoruns file you mention is the autoruns.arn file, save it and put it somewhere (eg dropbox, gdrive etc) and PM me the link or put in a zip and attach to post here.
I think we have eliminated the hardware options
Save to Disk and Add Books typically create directories, whereas a conversion won't - which suggests the problem is related to directory folder creation - which is curious.
Could you grab a text file (anything will do), anything with extension TXT and add it to an existing book that does NOT have a TXT format - just drag drop it onto the book details panel (where the cover is). That will add the file to an existing book folder - assuming it works.
If that works go into the library folder and create an author folder (eg ZZZ Author) if that works create folder in there (eg ZZZ Book), if that works copy a couple of files into the ZZZ Book folder. That's essentially what happens when you add a new book for a new author - if it works then ??? then how come calibre has a problem. If it doesn't work then at least we'll know its not a specific problem with calibre. Don't forget to delete XXX Author etc.
Another thing to try : create directory at the root of your system drive called something like CalTemp, then create an environment variable CALIBRE_TEMP_DIR that points to that directory (see
[User Manual] Environment Variables
BTW I stumbled on a gadget last week called Rapid Environment Editor, its free, it sure beats that fiddly thing MS provide.
Rapid Environment Editor - Screenshots
Given that you get the crash with 64 bit calibre in Safe mode but not with 32 bit calibre in Safe mode suggests to me its not another program. Firstly the other programs that would be running in Safe mode are minimal, secondly they are there for both the 64 bit and 32 bit versions.
I'm running out of ideas
BR