that because something wrong with the language file it self. to debug it:
1. at joomla administrator > global configuration > system and set Debug Language = Yes
2. goto frontend > Jomsocial page
3. have a look at footer there are the problem with this line:
JROOT/language/de-DE/de-DE.com_community.ini : Fehler in Zeile(n) 1208, 2493, 2559
have a look before and after those line and make sure the language is valid, mean