This is strange. I have created two almost identical newsletters in test.wikipedia and MediaWiki.org. When I click "Manage", it works on test.wikipedia, but not in MW.o. I believe I have the same permissions in both wikis (administrator and publisher of that newsletter).
- Works: https://test.wikipedia.org/wiki/Newsletter:Newsletter%C2%B2
- Doesn't work: https://www.mediawiki.org/wiki/Newsletter:Newsletter%C2%B2
Both pages look the same, as an admin/publisher I see the same buttons. However, clicking "Manage", in MW.o I stay in the same page.
Introducing the URL manually, I also end up in the same page: