Are you doing this on a production server? With large UBB.C -> UBB.T imports I'd generally only do it on a dedicated server which doesn't have much of anything else going on...
Classic in itself was a resource hog, and basically what you're doing is reading every file ever made in classic and then writing it to threads; there's just so much that can go awry and wrong during an import from that particular product...
dedicated server with a ton of horsepower and no other activity save the import/conversion.
i would expect to see the mysql db updating, but that doesn't happen at all.
soon as it begins the post import is when a popup asks what to do with the importer script. the mysql activity goes dead.
