Even with analytics off (carbonioSendAnalytics set to false, carbonioPrefSendAnalytics set to false for both COS and all accounts), login sets PostHog cookie for the entire domain: if Carbonio is located at "webmail.example.com", cookie is set to ".example.com", resulting in the cookie being accessible to all subdomains.
This cookie can grow too large under unknown circumstances and trigger "cookie too large" errors elsewhere in the domain (in other web apps).
Update: to clarify, PostHog's cookie doesn't break the default header size itself, but in combination with other large cookies from other apps.
I've disabled PostHog's initialization in the frontend JS code. It would be nice to have an upstream fix in carbonio-shell-ui to avoid this pollution in the future.