After troubleshoot long hour ago, it seems like these language translation string
ä,Ü causing the issue from this language file ->
JoomlaFolder/administrator/language/de-DE/de-DE.plg_app_user_discuss.ini
APP_USER_EASYDISCUSS_INVALID_TITLE="Bitte wähle eine Überschrift"
APP_USER_EASYDISCUSS_INVALID_CONTENT="Ohne Inhalt ist es schelcht. Bitte füge einen Inhalt hinzu"
Because these string is load from the ajax call, it seems like it conflict when you click on the filter link.
I've help you temporary change to normal character, so it should work fine now.
By the way, I will log this issue on our issue tracker, we will see if we can find out a better solution to solve this in next release version.