invalid action is triggered when ?action=<action> doesn't meet any of the cases handled by app/routes/route.json.php, this could be happening because outdated / edited app/routes/route.json.php so the action is not being handled by the system.
The other alternative is that JS is not sending the right action, for which case you should provide the HTTP request parameters to check if the thing is using the right action.
As I'm unable to reproduce the error and since others aren't reporting this, this doesn't qualify as a bug.