The query intitle:/.*lav/ https://cs.wikipedia.org/w/index.php?title=Speci%C3%A1ln%C3%AD:Hled%C3%A1n%C3%AD&limit=500&offset=0&ns0=1&ns100=1&ns102=1&search=intitle%3A%2F.%2Alav%2F does seem to fail producing Search backend error during regex search for 'intitle:/.*lav/' after 2525: runtime_exception: runtime_exception: Unreachable in the logs.

A quick look suggests that it might be caused by https://gerrit.wikimedia.org/r/plugins/gitiles/search/highlighter/+/refs/heads/master/experimental-highlighter-lucene/src/main/java/org/wikimedia/highlighter/experimental/lucene/hit/AutomatonHitEnum.java#189

It does fail only when setting limit=500 in the example query above and thus might indicate that the bug is dependent on the characteristics of the content being highlighted.