Caching issue on site - users can't logoff

I have a new issue that has started to affect my site where it seems pages are getting cached so they aren't updating when the user logins meaning some users will have to login a few times before they can see content. It is also causing issues with some users being able to logoff. They are unable to logoff at all and get stuck in a loop.

I have a video of the issues I'm able to share privately with the CMSB team.

As it's something that appeared I am assuming it has to do with some server settings (this problem wasn't evident before), however I've explored everything I can with the web host and they are unable to help as they don't know whats wrong

They are running opCache if that helps!