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Doing some quick research on this... I've seen several potential causes some could be on the user side and others are on the forum side. I'll start with the user side ones since that is the more likely since others aren't reporting it. I'll post a potential forum caused one afterwords.
One user ended up disabling their adblocker extension and the problem went away. I read elsewhere, they suggested disabling all extensions to test. If the problem clears up, enable them one at a time and test again to find the culprit.
Other protection software ( Antivirus, antimalware, firewall etc ) could also cause this if they block the token/cookie. Disable them temporarily to test. Remember to re-enable when done.
Another idea... try using a web browser you don't normally use when browsing the forum ( ie Chrome, Edge, Firefox etc ). If the problem was one of the protection softwares... testing using a different browser won't tell you anything unless you first tested by disabling the protection software first.
Now for the forum side of things...
Really, unless there was a recent update or something, most causes were from minor corruptions in the MySql database and performing a repair usually fixes the issue if caused on this side of things.
The other issues I saw on this end were also caused by database problems but they were caused after importing data and/or creating a new instance using a script instead of the standard installation method. These last ones were caused by missing database fields. I think this is the least likely but wanted to mention it just in case.