Amazon Mechanical Turk External Error Error

I am trying to implement an external question in Amazon Mechanical Turk. Everything works fine, except for the input submit

, which keeps returning the following error:

There was a problem presenting the results for this HIT.

This HIT is still assigned to you. To try this HIT again, click "HITs Assigned To You" in the navigation bar, then click "Continue on this HIT" for the HIT. If this issue persists, you can contact Requester for this HIT using the Contact link above.

To return this HIT and continue working with other HITs, press the "Return HIT" button.

I tried everything I could to get the job submit button to work and check all the posts I could find on Google and this website, but nothing seems to work.

This is the code for the submit button on my website:

<form name="hitForm" id="hitForm" action="https://workersandbox.mturk.com/mturk/externalSubmit" method="POST">
<input type="hidden" name="assignmentId" value="<?php echo $_REQUEST["assignmentId"]; ?>" />
<input type="hidden" name="hitId" value="<?php echo $_REQUEST["hitId"]; ?>" />
<input type="hidden" name="workerId" value="<?php echo $_REQUEST["workerId"]; ?>" />
<input type="submit" class="btn btn-primary btn-lg active" role="button">
</form>

      

        
        
        
      

    

The user has to click the submit button on the form as soon as they are done to submit the HIT and receive their payment. I tried to submit the form more and with less information, I tried hardcoding the information (of a user who is currently testing HIT) into the form, I tried to use a hyperlink with user data, and I tried to submit a task from different users, from different computers. from different networks, etc.

Any help on this error would be greatly appreciated.

Edit:

I've tried what Thomas said, but I still get the same error message. Now my form looks like this:

<form name="hitForm" id="hitForm" action="https://workersandbox.mturk.com/mturk/externalSubmit" method="POST">
<input type="hidden" name="assignmentId" value="<?php echo $_COOKIE["PlayerUserName"]; ?>" />
<input type="hidden" name="foo" value="" />
<input type="submit" class="btn btn-primary btn-lg active" role="button">
</form>

      

        
        
        
      

    

And yes, I am working on the requester sandbox to make sure my entire HIT is working properly before opening it to normal mechanical turks.