For some reason, I can't reply to Max_aaa's question directly.
> How do you guaranty that nothing in the messages being handled by the server is "sensitive personal data".
You guarantee it by reading the rest of GDPR. It defines sensitive personal data separately than personal data. Sensitive personal data is defined by GDPR to be things that can be used to discriminate against the individual, such as race, ethnicity, religion, health information, credit information, age, etc.
EDIT: And what I mean to say is that if the messages aren't passing through the server or being stored on the servers, then the only info being handled by the server is the meta-data including IP address, which is not included in GDPR's definition of _sensitive_ personal data.
