So I had all sorts of problems logging on?camer up with those time out errors JP mentioned in his OP, then when I logged out to log back in it threw up those session verification errors. Cleared cache, cleared cookies, cleared browsing data, tried it on Chrome, Safari, Edge.
finally what worked was going into incognito mode, which was a fix I found on the Simple Machines Forum (SMF) message board.
If my one is reading this and can?t post, try doing that. Incognito mode is on chrome but all browsers have a privacy mode. To me it suggests an issue with the cookies that the site is sending to our browsers in non-privacy mode. Maybe worth having a look at.