OX EAS (Microsoft Enterprise Active Sync) does not work when apache2/force_https is enabled. Reason is that OX USM (Universal Synchronization Module - framework that is used by OX EAS) connects to the frontend Apache (com.openexchange.usm.ox.url=http://localhost/ajax -> see /opt/open-xchange/etc/usm.properties) but will also be forced to use https (because localhost is not excluded from the Apache Redirect) which will then fail.

Oxtender2 for MS Outlook most probably will also fail (because it also relies on USM).

A possible fix will be to add localhost from also beeing excluded from the redirect in /etc/apache2/mods-enabled/ssl.conf:

RewriteEngine on
RewriteCond %{HTTPS} off
RewriteCond %{REQUEST_URI} !=/server-status
+RewriteCond %{HTTP_HOST} !=localhost
RewriteRule ^/?(.*) https://%{SERVER_NAME}/$1 [R=301,L]