I'm not really sure what caused the error, but when I start a thread title with "Å" I am getting a 404 error when I try to go to the thread. However, I don't think this is caused by the addons I am using. I am using XenForo version 1.5.0.
Have you got a URL to the thread? The actual thread title in the URL has no bearing on whether it's a valid URL or not as the thread ID at the end is there for that.