The language files aren't loading for plugins in backend likely due to the below bug.
forge.joomlapolis.com/issues/5609
Alternatively it could be due to out of date language plugin. Please ensure you're using the latest German language plugin found below.
forge.joomlapolis.com/projects/lan-cb-de/files
Specifically the following string.
'SEARCH_ATTENDING_a45168' => 'Teilnehmer durchsuchen...',
Is found in the below file.
components/com_comprofiler/plugin/language/LANGUAGE/cbplugin/cbgroupjiveevents-language.php