Youtube gave error 405 when i tried to log back in from the home screen. merely clicking another video showed that i was logged in. When i went back home i’d be logged out again. This is on Windows 10 with the latest mozilla firefox. Started when i updated to Kaspersky total security 2021. It persisted after i disabled internet protection. The problem does not occur in MS edge.
When I uninstalled Total security 2021 and rebooted the problem went away. The problem did not reoccur when I installed total security 2020. The old version is 126.96.36.1995(k).
Hopefully the bug will be removed in a later version of total security 2021. I got the link for the old version from Kaspersky chat in support.