Thanks to Jean-Pierre Reyt's heads up (and patience), I have identified a well hidden bug in the One-Click Templates add-on. It's related to the permissions of the temporary directory in which the add-on downloads and unzips a template. Weirdly, that bug did not happen in the past. I am currently working on a fix, stay tuned.
Update:fixed, email sent to all purchasers, bluegriffon.com updated