<meta charset="UTF-8" /> is probably causing your German language issues
This happens when you have several posts stored within your database as "ISO-8859-1" and then attempt to display them as "UTF-8."
The simple solution would be to revert your page charset header code from "UTF-8" back to "ISO-8859-1"
Instructions are at:https://www.ubbcentral.com/forums/ubbthreads.php/topics/256672/re-what-could-this-be#Post256672
If you are starting a brand new forum installation, its recommended to begin with UTF-8.
But if you already have a forum in a non-English language which has been running for several years, you should probably stick with the older "ISO-8859-1" charset.
There is a known "issue" within the 7.5.9 upgrader script which automatically updates your charset language string to "UTF-8". For most sites, this is a non-issue. But this may cause an issue for sites that are non-English, or where users choose to type up their documents in older/obsolete versions of Microsoft Word, then copy-paste to their posts.