Probably the letter "ø". A lot of non-english characters can be problematic in IT.
If you check the URL on caches on geocaching, you will see they remove Æ,Ø, Å and other non-english characters, probably to prevent issues with people not being able to type them out.
I have encountered many webpages that have had issues with those letters, and I am betting it is the issue here aswell.