The tag cloud widget wouldn't display my special characters (German Umlauts such as ä,ö,ü) though all templates are utf-8.
To fix this, go to /blog/code/widgets/TagCloudWidget.php and in Line 56:

replace:
$theseTags = split(" *, *", strtolower(trim($entry->Tags)));
with
$theseTags = split(" *, *", mb_strtolower(trim($entry->Tags)));

Aside from the tagcloud widget there was yet another function/module/place where i had to replace strtolower recently. But i forgot. Maybe it was a custom limitCharacters function. Anyway. Maybe it's a hint for resolving garbled umlauts that appear on other places.

Thanks Ralf, you made my day ;-)

cheers, vr

derralf good note - this should probably be changed in the module so if it hasn't been raised yet make a ticket on open.silverstripe.org for the blog module.