push to repo using mq broken via ssh

I have two repos that use mercurial's patch queues. Pushing to them via ssh has been working fine for ages, but stopped working on or after 27 March. Now I can't push the main repo via ssh, but can push the related .hg/patches repo.

Pushing via ssh gives the error

abort: no suitable response from remote hg!

I use the hgrc file for the url to push to, and this hasn't changed, so I know I am pushing to the right place.

Non-mq mercurial repositories are working fine via ssh.

Pushing via https DOES work, but is a big pain.

Both repos are team-owned, in case that is relevant.