By 15 million rows I meant the index table size.
Okay, this sounds like a serious issue with the actual database, I suspect fragmentation or data corruption. I strongly recommend doing table repair commands, checking the memory usage and the available space for the database.
It might be the best to completely delete the wp_asp_index table, deactivate and re-activate the plugin so it is recreated automatically, then doing a full database repair, as well as optimize table commands on all of the database tables. Those are crucial. There is no other reason for the plugin not to work on this server, if it worked just fine on the other – unless there is an underlying issue on the database/server layer.