Okay, I got it working now.
I tried Firefox, and I didn't add any kind of special characters anywhere. Wrote my phone number without the +46 (swedish code), wrote "a/o" instead of our "å/ä/ö/", etc. Not sure exactly which part the site didn't like, but it worked now at least.
Seems like...
I just tried donating, but I've no idea if it worked, because I got a "page unavailable". So I'm not sure if money was actually transferred or not (and I probably won't be able to see it until after the weekend when the drive is over). Did what I could, so I can't really try again and...