Could this be a macOS Big Sur problem as I updated my MacBook yesterday?
Yes. Oddly, I had no trouble with the Photoblogger update, but updating a couple of my own skins failed miserably - see screenshot (I couldn't even get to the buttons to close the error window - had to quit jAlbum).
I am also on Big Sur final. Just to see if JDK 15 could hamper jAlbum I removed that temporarily, but it had no impact. The result of double clicking on PhotoBlogger.jaskin gave the same error. I added now a screenshot of the the whole window.
Thanks for reporting this. It's a apparently either a removed "Core Services" library in Big Sur or some security restriction that has been introduced. I see no documentation on such from Apple so we have to wait a bit, but I've made a workaround in this beta: http://jalbum.net/download/beta/jalbum-core.jar
What doesn't work now in Big Sur is for jAlbum to move items to the recycle bin. When this fails, it will now simply delete the item instead. If you keep the logging level at "FINE" under Preferences, then jAlbum will also print a message about this error to the system console.