No Space on C, GUI does not work
I know you don't bother about a setup with no space on main hard drive but I report you what happened to me.
I have set up calibre_temp_dir to another hard drive.
While working with calibre the C drive exausted the available space. (Not dependig from calibre)
Then I have open the modifiy metadata in group and then search/modify metadata. I have selected "use regular expression" and then search for title and modify identifier (isbn).
Then an error message appeared: GUI error, Disk full, or something like this.