The remark about RFC5335 is not relevant. The example From: headers in the report are RFC 2047 encoded, and are valid as the encoded headers contain only us-ascii characters.
In any case, the report talks about LP rejecting mail. Is this mail to a Mailman list? If so, what happens to the mail. Is there an actual rejection notice? If so, what does it say? What's in Mailman's various logs, in particular 'vette' and 'error', relating to these messages? What are the lists settings for Privacy options... -> Sender filters? Is this user in accept_these_nonmembers? What is generic_nonmember_action?
This very well could be bug 702516 as the actual decoded From: header is
From: Claußnitzer, Ralf email@example.com
which does contain an unquoted comma which is the issue of bug 702516. If this is the issue, the reason it works if the "ß" is replaced with "s" is the real name then contains no non-ascii, so the users MUA simply quotes it rather than RFC 2047 encoding it.