I am interacting with a user of the SBF forum, who has the same problem (on OS X, though.) and he seems to confirm that the problem relies on the attempt to create a tmp directory to unpack the EPUB.
I am planning to have a better look at the issue tomorrow, and I guess the solution involves taking advantage of the standard Calibre function that gives you a temp dir.
Another user confirmed that the plugin runs well on another Linux machine, with Calibre version 0.8.61 (which is different than mine).
EDIT: Terisa, would you mind 1) uninstalling the plugin; 2) installing from this file
; 3) restarting Calibre; 4) trying to apply your ex libris? It should work now.