Something's definitely wrong on the back end. What error are you getting?
I'm not getting any kind of error. It just doesn't download stuff at all. Here's a screenshot to show you what I mean:
Oh that's even worse. I'll see what I can do but I'm hoping fixing the other issues will fix whatever this is.
Turns out Gelbooru updated their software at some point and broke the image downloading code pretty much completely. I'll be uploading a new version with a fix for that and some other bugs as well as some new features in a few days. Sorry for the inconvenience.