Odd. When I open the basic add books dialog, the file type selector box lets me select from Books (most book file types) to all files (*.*).
Can you give a better description of how you are attempting to add books? Just clicking on the Add Books icon? One of the options from the dropdown menu? Add from folder and subfolders?
Which antivirus are you running? I've seen a similar error when AV was blocking calibre's normal operations. See
https://bugs.launchpad.net/calibre/+bug/1899778
Edit: If your AV allows, you could try excluding the calibre2 directory and it's subdirectories.