This is a kinda guesstimate, here, after a suggestion by another member.
Admin > Search > Search method
Try setting that to "Full text index" and click the "Remove fulltext index" link thingy.
Then, try the conversion again and rebuild the index after the conversion. As we don't know which table has the fulltext thing hassling us, that's a bit of a wild guess. But, I assume you backed-up, before attempting the conversion... So, if this causes anything goes "PHUT!", we'll be OK. (I doubt it will, though)