it's the spam filter. The only banned words are a couple of slurs because they encourage low quality posting, and words that are common in the worst kind of spam. Outside of that, the filter blocks characters that would be in urls, but only sometimes, and frankly I can't figure out how it works either because it should just be regex but things which shouldn't effect it are effecting it and it doesn't make any sense.
I'm going to keep experimenting with the filter and try to make it less dogshit, but yeah, sorry. For the time being, avoid slashes and colons and things like that. They're not even supposed to be getting blocked, and the filter lets them through sometimes and I can't figure out what's causing this. Either way, I hope you can appreciate that this annoyance is partially necessary against having egregious spam advertising heinous shit on my website which I obviously don't want. But I have been and am continually trying to make the filter less shit so once again sorry.