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.