For the record, this was the response from the FSF: https://www.fsf.org/blogs/licensing/agpl-is-not-a-tool-for-taking-freedom-away

In summary, you can add additional terms to disclaim warranties, include some extra notice/attribution, etc. but the terms can’t really require something that effectively limits the rights explicitly granted in the AGPL (like… you can’t require the inclusion of a trademarked logo, since it would effectively restrict anyone who doesn’t have trademark rights), the license explicitly allows licensees to ignore/remove any terms that impose additional restrictions like those.

In the end, OnlyOffice changed the license headers, removing the contradictory restriction.

They also added additional terms section in the license file, with some things that were not explicitly requested before (and that it’s still contested whether they fit what the AGPL allows as additional terms). But they only did that later, Euro-Office is still using the previous version of the license that did not have those new additional terms.