Cool, I just tested in your live site, after disabling ServiceWorkers, and the problem persist.
I think it’s an error on the reverse proxy configuration, because I can repro the error with a simple curl:
➜ ~ curl -v https://i.materialise.com/forum/clicks/track\?url\=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DRaiLd3QBmC0\&post_id\=589\&topic_id\=388
* Trying 54.208.178.103...
* Connected to i.materialise.com (54.208.178.103) port 443 (#0)
> GET /forum/clicks/track?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DRaiLd3QBmC0&post_id=589&topic_id=388 HTTP/1.1
> Host: i.materialise.com
> User-Agent: curl/7.55.1
> Accept: */*
>
< HTTP/1.1 302 Found
< Cache-Control: no-store, must-revalidate, no-cache, private
< Content-Type: text/html; charset=utf-8
< Location: https://i.materialise.com/watch?v=RaiLd3QBmC0
< Set-Cookie: GaClientId=84c67115-16e3-4963-887a-b4455b6fe63d; expires=Fri, 19-Mar-2021 13:56:40 GMT; path=/
< Set-Cookie: GaClientId=84c67115-16e3-4963-887a-b4455b6fe63d; expires=Fri, 19-Mar-2021 13:56:40 GMT; path=/
< Set-Cookie: lang=EN; expires=Mon, 17-Sep-2018 13:56:40 GMT; path=/
< X-Frame-Options: SAMEORIGIN
< X-XSS-Protection: 1; mode=block
< X-Content-Type-Options: nosniff
< X-Discourse-Route: clicks/track
< X-Request-Id: 1055bd86-75bd-4020-8840-0b4f6a18f1b3
< X-Runtime: 0.006256
< Referrer-Policy: no-referrer-when-downgrade
< P3P: policyref="/w3c/p3p.xml", CP="CAO CURa ADMa DEVa TAIa OUR BUS UNI PUR COM NAV INT CNT STA"
< Date: Mon, 19 Mar 2018 13:56:39 GMT
< Content-Length: 109
<
* Connection #0 to host i.materialise.com left intact
<html><body>You are being <a href="https://www.youtube.com/watch?v=RaiLd3QBmC0">redirected</a>.</body></html>
Look at the Location header in the response.