Cannot login after Perch 2.8.34 update

I'm having a problem with Perch login after a site's 2.8.34 update. I'm immediately logged out and the logout URL ends with %2Fcore%2Fapps%2Fcontent%2F. I've seen this before and I know that this is a sessions problem but the web host tells me that sessions are being written in the appropriate location.

I solved this issue before by an upgrade to Perch 3. Is that the only solution?

Thanks

The PHP handler is cgi. Is that relevant?

Nothing.

I can see sessions created in the /tmp folder location

Yes, have done that. See my previous reply

Login just redirects back to the login screen.

The web host has said that sessions are OK but my client is following that up with them

I'm in the process of setting up a staging site before a potential upgrade to Perch 3. That's all I can think of to try but I am a bit concerned that the blog update in 2.8.34 has not completed. Will that affect Perch 3 upgrade?

Local site is fine. There's no problem there.

I don't know re caching. However, have tried login with VPN and/or Private Window and there's no change.

I'm using regular Chrome and there's no ad-blocking as far as I know.

In the end I got round this by updating to Perch 3.1.5 on a sub-domain staging server and deploying that to live. Not totally sure if there was an issue with the server or with a Perch update prior to Perch 3. Anyway, that's how it was resolved.