I ran into this issue
myself, so a heads up if anybody else faces anything similar:
If you have an encrypted Fairphone2, the upgrade from Android 6 to Android 7 may fail to decrypt the device after installation. DO NOT
reset the phone, although the device will tell you that's the only option. You can reinstall Android 6 manually, and your device will be just fine. Follow the instructions in the link
. You can then install the update by disabling startup security before
starting the upgrade.
In any case back up your device before you try!