The maxBooleanClauses setting in SOLR's core config (solrconfig.xml) is very important for large sites! If you have category-level roles, and large numbers of courses within the categories, Moodle generates a horrifically long search string (contextid:(0 OR 1 OR 2 OR 3 OR 4.. etc etc etc)).
SOLR just fails if this is over 1024 by default, and the error page isn't pretty for users!
I think Moodle should check what the value of maxBooleanClauses (if that is even possible via the API, I am no SOLR expert) and not attempt to query more context ids than that (fail with a proper moodle_exception, rather than that nasty long string and SOLR error).
To check this out, create 1025 courses in a category, and give a non-admin full access to that category, then try to search.