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!
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fairphone #
upgrade #
warning