Not that, it will help, but I’ve just reinstalled the whole website. So in other words removed the whole of WordPress and Zenphoto and installed them all over again. So a bit like reinstalling Windows from square one. Doubt with help, but fast running out of ideas!
Only been working on it since, getting out of bed at 9am! So want to cheer me up?!?? Then comment and rate the photo to left!
I hope it alleviates what might be causing the issue. Sorry to hear, wish I could help.
Considered a different hosting company or a new build? Sites get a bit patched together over the years and sometimes need a new one.