Trouble with Amember integration and 3.8.4

[Hilary]

I wonder if anyone else had this problem and has found a solution?

The original setup -

I have Amember and Vbulletin fully integrated, so Amember handles all signups and logins for Vbulletin. New members sign up in Amember and are added to the relevant Vbulletin usergroup; when they sign in to Amember they're signed in to Vbulletin. Fine.

Normally, I run Vbulletin's upgrade and Amember isn't affected at all. This time, upgrading from 3.8.3 to 3.8.4 patch 2, the login is broken.

Things that still work:

• database integration - that is, new members (Amember) still become new Vbulletin users
• Vbulletin's own login - that is, I can still sign in as administrator through Vbulletin admincp, independently of Amember
• Amember login - that is, people can log into Amember itself and everything it protects except for Vbulletin.

Thing that doesn't work:

• single login. People logged into Amember are not logged into Vbulletin.

This is all very weird indeed. Minor upgrade of Vbulletin, no changes made to Amember.

Anyone already encountered and fixed this? Or have any inspired suggestions? I'd be pathetically grateful...