We found that the Multisite rule "uri-prefix-not-empty" did not match correctly the multisite definition. The same configuration works for 5.6.1 but not in 5.6.3.

I attached the multisite configuration and the page tree screenshot.

The problem was that the page "/de" was not reachable and producing 404 because "unibas-newsletter" site definition was mached instead of "fallback".

The Problem is linked to the page having the same name than the locale ("de")

I fixed the issue by modifying the Class NotEMptyUriPrefixMatcher. (see attachment, Class and rules config)