'./wikidb/searchindex' is marked as crashed and should be repaired
I experienced this same behaviour when upgrading from MySQL 3-4 and from MediaWiki 1.6->1.11. I think I've fixed it (hopefully permanently) by dumping the table, then dropping it, then re-creating it again. I found that repairing the table worked, but the error would re-occur. Hope this works for you.
Peter