Our problem was and still is that the SOLR-search does not really work well under usabililty aspects. The display and logic of search results is rather misleading. E.g. the given context
of a search result in a book (activity) leads to the first page whereas the term itself could be anywhere in the book. Because of that we changed as a minor action (as we thought) the display of search results which led to the error I described in my original post. Yet we haven't fully understood the relation of search areas and the display of search results and the consequences of changes of the config.php we did in order to solve our problems.
TL;DR: Our problems ocurred because we changed the config.php.