Minor Description
CrawlDbReader reports syntactic errors in Jexl expressions only in task logs (hadoop.log in local mode) and continues as if there where no Jexl expression set. It should report it more verbosely and probably also fail the job, at least, if the error can be checked at job start.
In my case a trivial error (score > .9 instead of score > 0.9), the crawlDb was just left unfiltered.