I've been trying to add new categories to my gallery but with no success. I deleted one to make sure it wasn't a space issue, despite the fact that I'm nowhere near 20, but still no luck. I don't get an error message; nothing at all happens. Any idea what could be the problem?

You can't. When they changed the shops to be able to edit categories, a glitch creeped in. Hopefully it iwll be fixed soon.