This is just for information, maybe it helps someone else solving update issues.
After updating Fairphone Open 17.04 to Android 6 the phone rebootet, optimized its apps and startet. But after some seconds it froze, no reaction to any kind of input. After about one minute it restarted, just to freeze again after some secods. And so on …
I saw a notification asking me how to use my SD card. A popup asked for permission to read external memory for the app Orbot. Allowing Orbot to use external memory succeeded after some reboots, but freezing/rebooting went on. Maybe the problems are a result of a yet unavailable SD card. Trying to add the SD Card didn’t succeed in the limited time until the next freeze occurred.
I tried to uninstall some apps relying on the SD card (Orbot, LibreOffice Viewer) and unchecked UnifiedNLP providers which have their data on my SD card.
After that the freezing/rebooting is gone, now I’m trying to get used to the new OS.
@Fairphone: Thanks for the Update!