It's sadly not enough to simply mark the skin's filter method as synchronized. The filter needs to follow the JavaBeans specification and not rely on states between each filter invocation. I commented in the other thread. Please upload the source code for your filter there and I'll see if I can figure out why it's not working properly. Also tell what incorrect behavior you get.
Well it now works wit your change to jalbum-core.jar; thank you so much! I have updated the skin and uploaded it with a note of how to update to 24.4.1 (ahead of the release) and also note that jAlbum 23 should work fine (with this most recent bug fix too).
Everything is in order on my part that I can tell. I just had to add Cloneable to the list of implemented classes. I don't know if it's necessary for the skin but it definitely seemed to be for the filters - but I kept it at both.
Really appreciate it all the more since you're on holiday. Enjoy the rest of it and stay safe!