TYPO3 Core

[FE][pageNotFound_handling] set to /index.php?id=200 (that is the designated 404 page). This is necessary for a multilanguage setup with several domain records.

With the change http://forge.typo3.org/issues/81031 a strict type comparison is introduced in the error handling process, that now skips the host determination, if none is given. For my case, the host part is obviously not only empty, but not even present. The new strict check skips the host determination, so we stick to a relative URL. This is then redirected some lines down, resulting in the markup of that page being part of the URL.

Description is pretty difficult, sorry :/.
A simple isEmpty check instead of the strict type empty string one, solves the problem.