During a crawl, the ManifoldCF > Status and Job Management page shows a 500 Error response.
Normally this is due to MP3/MP4 files being crawled. Solr, by default, cannot crawl multimedia files successfully.
This article addresses this issue most of the time. If it does not, set the MP4/MP3 items to not searchable for testing. That can be done through the workarea, or SQL after the appropriate back ups are taken.