Setup / situation:

• Using fallback site defintion, meaning NO specific site defintion is mapped
• Content i18n in (fallback) site defintion is enabled -> single tree multi lang is used (dialogs react etc)

Problem description:

A page using "single tree multi lang" content i18n returns for NON-default-locales a 404

Problem source:

The 'NotEmptyUriPrefixMatcher' in the multisite module.
Remove it and it works.

Most likely the bug came in via this ticket: MULTISITE-80

Behavior hypothesis:

The system has a matching rule as the 'NotEmptyUriPrefixMatcher'  returns true.
-> The fallback site defintion is not used (fallback is  used when no matcher rule matches)
-> the i18n configuration enabled=false is used instead of enabled=true
     (i18n enabled=true means -> /de/ is just a flag but not a page, set local and remove it form currentURI
      i18n enabled=false means -> /de/ is a flag and a page, set local and don't remove it form currentUR)
-> the system thinks the /de/ flag is a real page and tries to fetch it -> 404

Reproduce:

Generally:
Create on top level a page which used i18n enabled dialogs.
Add non default content and try to render it -> 404

Convenient:
Copy the page /travel/about/company to top level and switch to de -> 404

Test done on bundled versions:

Magnolia 5.5.9  works (multisite 1.2.6)
5.6.1 works (multisite 1.3.0)
5.5.10 does NOT work (multisite 1.2.20)
5.6.2 does NOT work (multisite 1.3.1)