I create a blog post "test ä" (with a german umlaut).
After submitting it I try to add a comment.
Confluence complains about an error "The page specified does not exist" and shows the page with name "Add Comment to SPIELPLATZ:Test ä"

However if I look the post directly, it is corretly display as
"Dashboard > SPIELPLATZ > Pages > 2004 > April > 06 > Test ä".

Looks like an encoding problem to me.