You could try the TWRP way to completely wipe the phone and make a clean install from scratch … I found that to be very helpful and reliable when trying out different OSes.
Attention! This will remove the installed OS and all your data from the phone (which most things labeled “factory reset” on the Fairphone 2 will not completely do).
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Get TWRP running as recovery (skip this step if you have the latest version running already):
Download the latest TWRP image from the “Download Links” section and follow the “Fastboot Install Method (No Root Required)” section over at twrp.me.
Mind the image filename, they give a generic one in their instructions.
(Let the phone stay connected via USB afterwards, that will be needed again later to transfer a file). -
Download the file from the “OTA” section of the Fairphone Open OS downloads (not one of the OTA switcher files) as a preparation. Transfer follows later.
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Boot the phone into the TWRP recovery.
In TWRP:
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Settings - uncheck “Zip signature verification” - back to main menu
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Mount - check “System”, “Cache”, “Data” and “Micro SDCard” (if available) - back to main menu
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Wipe - Advanced Wipe - check “Dalvik / ART Cache”, “System”, “Cache”, “Data” and “Internal Storage”, do not check “Micro SDCard” (if available) - Swipe to Wipe - back to main menu (if it doesn’t return automatically)
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Transfer the OTA file downloaded earlier to the microSD card (if available) or to Internal Storage (should work, but I never tried that myself, I use a microSD card).
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Install - Select Storage: (depending on where the file is) “Micro SDCard” or “Internal Storage” - select the OTA file from the list - Swipe to confirm Flash - when finished, reboot (this reboot really might take a while, so be patient)