When including expressions in the END URL field there are usually errors, when you use an if loop to present different URLs based on previous questions for example. Something like:

{if(Participate.NAOK=="A1", "http://www.google.com", "http://www.bing.com")}

This is not a new problem, I recall to have seen this for years but it is still prevalent in 5.1.18+211101 and I am updating to 5.2 now and I guess this is the same.

The current workaround is to create an equation question that determines the URL to redirect to and then pipe it into the END URL field via {equationquestion}.

However, in my opinion this should also work directly in the End URL field, but there it creates some HTML code (spans, etc.) that cause an error on the end page.