TL;DR … a few thoughts about going back to Fairphone OS, if you just wanted to try /e/ temporarily …
- Obligatory disclaimer: Be sure about your backup contents or syncing status. TWRP will not backup Internal Storage, as usual.
- When restoring a TWRP backup, don’t restore every partition, probably stick to Boot, System Image, Vendor Image and Data .
- If restoring doesn’t result in a booting OS, instead focus on getting a stock Fairphone OS running again somehow, either via installing a stock package via TWRP, or via flashing a stock image dump with fastboot, or via changing the A/B slot and do a factory reset (which should bring back the Fairphone OS version before the last update, if that slot was left untouched since then).
I think when installing /e/ again, I might change the A/B slot beforehand to simply keep the current Fairphone OS I did get the backup from and install /e/ on the other slot. - Once the stock Fairphone OS is running, set the same unlock method you had before when you made your backup.
I found that to be necessary to restore my data partition when I initially played around with TWRP backup and restore on the Fairphone 3. - Before restoring Data, make sure the stock Fairphone OS is updated to the same build your backup was made from.
If you’re interested in why …
I took /e/ for a quick spin yesterday just to see whether I could install it (which is really easy if you are used to fastboot already), and whether it seemed to work well out of the box (which it did), and with the bootloader left unlocked (which it did), and if MyPhoneExplorer would work to restore my contacts and messages and stuff (which worked), and whether I could go back to Fairphone OS … which then was, of course, an adventure, because I was feeling adventurous 
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Before installing /e/ I made a full backup in TWRP of my Fairphone OS A.0118 installation, I made a copy of my Internal Storage, and I synced what I could sync with MyPhoneExplorer.
I did backup all partitions in TWRP, because for installing /e/ you flash more partitions than this here recommends for a backup …
… so I wanted to be on the safe side and see how it goes.
When I was trying to get back to Fairphone OS I just restored all partitions and ended up in the OS not booting … an effect I know from the Fairphone 2, with the remedy on the Fairphone 2 being an install of the fitting OS build over the restored installation. But no can try, because I’ve seen no A.0118 stock package or stock image dump yet.
So I thought let’s see what’s there on the other slot … changed the slot … had to factory reset … and it happily booted A.0111 (as expected from the A/B update workings).
I set up the screen lock with the PIN matching to the one I used before I made the backup and then tried to only restore my data partition in TWRP, which failed with the beloved “extractTarFork() process ended with ERROR: 255” which translates to “Something went wrong, but I won’t tell you what exactly, because I don’t like you!”
So I updated the A.0111 to A.0118 via the Updater, which worked, and tried again to restore my data partition, successfully this time.
A.0118 booted and data was back. I copied Internal Storage back and synced my stuff over with MyPhoneExplorer and everything is just working fine as before, just on the other slot now.