I’m getting the same issue. Both on PHP 8.0 and 7.4. I found it because upon hitting “New Post,” I was getting a fatal error. When I turned debugging on, I saw the same token message.
Disabling the Optinmonster plugin removed the fatal error.
I am on the latest version of the plugin (Version 2.6.5), which was just released a couple of days ago. I suspect this new version introduced this issue.