This is so weird. I am not sure what would cause this. But apparently you can't insert into feedsub (even though it's an entirely new server and remote profile) and you have problems verifying the subscription for some reason.
Not sure where to start debugging this. I'd just throw a bunch of common_debug(...) calls in HubSub->verify for example and see what each important variable is at each specific line (and especially whether it runs ->update or ->insert).

Same thing with the FeedSub class in the end of the ensureFeed(...) function. What is it trying to insert/update and with what (full) SQL query?