Hello,
Is this a legitimate email? What whith all the /cpsessxxxxx/.... request ?

My guess would be a routing problem in your nginx/apache conf.
SOGo only set cookie on path /SOGo and the server should only receive request on /SOGo/ too.
I think in your case the /cpsess.../ are actually send to SOGo + without the cookie as the url is not on path /SOGo. But SOGo seeing a weird request, decides to discard the auth cookie anyway.

Check in your nginw/apache that only /SOGo/ url are send to SOGo server and the others (/cpsess) are send to cpsess and not SOGo.