I'm using the current version of Zenphoto. I used the Albums -> [album name] -> Album Title: field to change the title, and the radio button to rename the directory to match (but lower case, hyphen separated).
After doing this, tags for the images within that album ceased to work. Checking the images, they had lost all their tags. The tag cloud still showed the correct total for those tags, but clicking it would yield no results. After retagging each image, the tag search works again, but the totals are now doubled. For instance where there were 35 results (one of which was a dynamic album) it now reports 69. There are in fact only 34 images with that tag.
Checking the database, there are twice as many rows for that tag ID as there should be. There is no indication which I should delete, why they weren't deleted when the tags were lost (which is the bug, I guess), unless I can just delete the 34 tags with the lowest IDs?