Zenphoto 1.6.8 plus up to date ZPBootStrap
Another "annoying but not a showstopper"
In some (but not all) albums (Admin --> Albums --> Images --> Images per page [top right]) the images per page can't be changed but brings up a "FORBIDDEN" error.
Bizarrely; if you go to an album where it does work, increase it to (say) 60, then go back to the original album, the "60" has stuck.
There is nothing obvious in the ZenPhoto logs - nothing higher than USER WARNING and those are "Invalid album instantiation: .... does not exist" with a timestamp bearing no relation to the initial problem.
Now trying to get hold of the Server Logs...
Try generally a lower number like 20 or 25. You might have ran into a form element submission limit on your server. On saving everything on the page is submitted and the page number may be the "casultiy" here. Respectively the internal token also submitted that tells ZP if you are allowed to do that action. There should be a kind of form submission error in the debulog though. But that is the only thing I can think of right now.
A somewhat bizare update...
One of our volounteer uploaders noticed that the only sub-albums which caused this issue had the word "Union" in the title (and, by default, in the filename). "Grand Union Canal", "Shropshire Union Canal" etc.
As an experiment, they changed both Title and Filename by dropping the last letter; "Grand Unio Canal" etc. And it worked!
As a "tidier" workaround, we've restored the Title (which doesn't seem to cause an issue) and changed the filename to "Grand U-n-i-o-n Canal" which seems to work as expected.
Is it faintly possible that the filename is not adequatly escaped and is throwing a MySQL error?