aCalDAV cuts connection with davmail server

When attempting to synchronize calendars with a davmail server, aCalDAV establishes the connection to the server on clicking the button "Sign in or register", but then cuts it almost immediately, as seen in davmail's log. No explanations are found anywhere, or is there a possibility to ask for additional logs (on the phone) somewhere in aCalDAV's settings? I do have rooted my phone, in case that helps getting logs.

The bugs happens even if the checkbox "Check SSL Certificate" is not ticked.

Davmail is a proxy from caldav to exchange (http://davmail.sourceforge.net/). If this issue cannot be solved, is there maybe an other way to sync the calendar with an Exchange server (on which most standard protocols are disabled... only Microsoft proprietary protocols do work)?

N.B. aCalDAV works alright with a davical server