Post Login Action: Can't get passback JWT validated "The session token is invalid: Missing or invalid standard claims"

I am having issues to complete my post login flow because I can’t seem to get my passback JWT validated on the /continue action.

There seem to be either a standard claim missing, or wrong. I couldn’t figure out cause I wasn’t able to extract a more detailed error message or debug it in depth with " Real-time Webtask Logs" . It would be at least nice to know what is either missing or what claim is wrong.

From the list shown here I ensured that all the fields are there. This is my business logic to build the new token by taking some fields from the sessionToken I received from the action-redirect:

    $tokenPayload= [
            "sub" => $tokenSessionFromAction->sub,
            "iss" => $tokenSessionFromAction->iss,
            "exp" => $tokenSessionFromAction->exp + 1000,
            "state" => $state,
            "optionalField" => "should show up in payload"
        ];

What am I missing here?