Stumbled into your forum subject during my search for an answer to my problem, which is exactly like yours.
In my situation, the Invalid token message only occurred when submitting the ask-a-question popup window for the first time in a session. Later in that same session, the message didn't return anymore.
Turning on or off System Cache didn't do the trick for me.
But turning off System SEF did! Also, setting Search Engine Friendly URLs to NO in the Global Configuration worked fine as well.
I had no problems in J251&VM202. Problem did arise in J251&VM204 and J254&VM204.
Also, problem(as well as popup window) didn't occur when putting off JQuery (working offline with Google JQuery ON).
It looks as if JQuery messes up the SEF URL of the former page (with the ask-a-question button)? (No experience with JQuery whatsoever.)