gwd-exit url values cause Publish to DoubleClick Studio error: could not upload files

[seik...@gmail.com]

Why are some urls in the gwd-exit causing the "Publish to DoubleClick Studio error: could not upload files".

This currently works:
<gwd-exit metric="helping-people-after-treatment-ends" url="http://www.macmillan.org.uk"></gwd-exit>

However if I add a trailing slash then save and publish I get the error and my creative is not uploaded

<gwd-exit metric="helping-people-after-treatment-ends" url="http://www.macmillan.org.uk/"></gwd-exit>

I cannot find any hidden characters, all I've done is add a trailing slash to cause this issue.

Weirdly I can set the url correctly in the JavaScript function
gwd.actions.gwdDoubleclick.exit('gwd-ad', 'helping-people-after-treatment-ends', 'http://www.macmillan.org.uk/aboutus/healthandsocialcareprofessionals/macmillansprogrammesandservices/livingwithandbeyondcancerprogramme.aspx', true, true);

If I use the correct url in the gwd-exit url attribute I get the publishing error as well.

The displayed error message doesn't tell me why the creatives cannot be uploaded or what's wrong.