Just re-read your post. You don't know that it has anything to do with Linux. The approved OS thing and the browser may not have anything to do with it. I could be due to your inactivity or some issue with your cookies. Any number of things.

I would try logging in with an incognito or private window. Who knows what they are doing. I think the supported OS/browser thing is mostly to cover them when people call having issues. Their support team doesn't know linux and also for the browser they likely do not test their site on Linux browsers. I know our team does but I work for a Linux company. That said, every team has limits.