It feels very much like some timeout, since it's always a little over 10 seconds. I'm uploading images that are at most a few hundred kB, so they're not particularly big in bytes or pixels.
I messaged the admin and tried to use redgifs but it's not loading the preview nicely like it would using RES, perhaps I need to link to the actual image, not the pretend one redgifs gives you from the share dialog.
You mean you've made a new account and you're posting there, or you're using federation and cross-posting? I know what kbin is, but I haven't checked it out properly.
No, I made a kbin account and was able to make a post on a magazine over there. I'm not able to log back into my account at the moment though, giving me a different error I don't know.