The button "Synchronize now" (Jetzt synchronisieren) does not work in any of my accounts (neither DavDroid nor ICalSync2 nor WhatsApp).
Latest discoveries: I go online when my server is not running.Then I turn the server on, and it does not synchronize. After a restart it starts syncing. My guess is, that when I am online, the FP tries to sync, but if the server is offline, then it fails and somehow gets stuck somewhere in the attempt.
I experienced that after trying the said button, when I scroll down the status bar and press the button in the topmost-right corner ( ), I cannot press any buttons. None of them work!
Can you reproduce the issue?
Edit: I am using FP OS 1.6
Edit2: Every other app works fine. It seems that pushing the "Synchronize now" button somehow froze part of the phone (maybe because DavDroid connects to my owncloud server (which is currently running))
Edit3: After a reboot sync starts itself immediately. However it's not very comfortable to reboot the phone every time I want sync to start. Also the "Synchronize now" button works now.
Edit[2015-06-01]: Still happens. I boot my OwnCloud instance, press "Synchronize now" and nothing happens. After a reboot of Fairphone OS 1.8.5, calendar and contacts synchronize. CC @paulkreuzer
Edit [2016-01-28]: I don't use owncloud anymore, but posteo, and it still happens. I have to reboot to force a synchronization.