at first I had the same error message ‘zip signature verification’ failed with the latest version of /e/OS (e-0.7-n-2020011737872-dev-FP2.zip )
My TWRP version was the one pre-installed on the FP2 : version 3.1.1-0.
Following what I read here I flashed the lastest TWRP version for the FP2 : version 3.3.1-1.
Then I could install /e/OS without errors.
However, for the record I had a lot of trouble having adb recognize my device.
Should this happen to you, what follows may help :
At first in the Fairphone Open OS (that was installed), I activated the USB debug mode.
This was not enough, I had to reboot into the OS to authorize my computer.
After my first unsuccessful attempt with sideloading, when trying to flash TWRP my device was again not recognized.
From trial and error I eventually found this solution :
I had to type on my computer :
adb devices (will start the adb daemon again)
This was not enough I had also to force reboot of the phone pressing the power button for 15 seconds.
Even after the successful installation of TWRP my device was again not recognized. I has to do the sequence again.
In the end after successful sideloading of /e/OS, when I rebooted from TWRP it said “are you sure ? No OS installed !” I did it nonetheless and /e/OS launched after some stressful dozens of seconds !
I am happy I was able to start /e/OS but was a stressful experience !