Happens often. I avoid SF sometimes because it is frustrating. i tried varios atforms, browsers and networks with the same results.
I know I have issues if I log on from 2 devices with the same IP. Switching one to another IP via VPN seems to resolve it. That is not always the issue however.
Please make sure you log out before closing the browser. Just to rule out multiple sessions from the same account causing an issue. Logging out prevents stale sessions from hanging around. It also frees up system resources.