Unfortunately, after the upgrade I am no longer able to reinstall TWRP.
Even when it is booted in download mode (home+vol-down+power or via advanced restart from developer options), the device does no longer show up in “fastboot devices” and thus can no longer be flashed… 
Also “adb root” is no longer working - the required option to allow root access via adb does no longer exist in the developer options
You need to unplug USB cable, boot in download mode, replug USB cable.
You mean “adb devices” ?
ADB usb Debbugging must be enabled in developper options settings or in the recovery-e advanced menu
That is expected if you choose the “stable” build,
Should work using “dev” build.
A Samsung does not respond to fastboot in Download mode. We have to use Heimdall or Odin in order to use Download mode.
When I use heimdall, I get an error. During the 2-3 sec. time between “Uploading RECOVERY” and ERROR a progress bar starts on the device screen. It stops proceeding when ERROR is printed.
heimdall flash --RECOVERY twrp-3.7.0_9-0-herolte.img
Heimdall v1.4.2
[…]
Initialising connection…
Detecting device…
Claiming interface…
Setting up interface…
Initialising protocol…
Protocol initialisation successful.
Beginning session…
Some devices may take up to 2 minutes to respond.
Please be patient!
Session begun.
Downloading device’s PIT file…
PIT file download successful.
Uploading RECOVERY
100%
ERROR: Failed to confirm end of file transfer sequence!
ERROR: RECOVERY upload failed!
Ending session…
ERROR: Failed to send end session packet!
Releasing device interface…
After that, the phone booted neither in recovery nor normally, but only into download mode (where the above error persists).
All other options ended in a screen saying that “An error has occurred while updating the device software. Use the emergency recovery function in the Smart Switch PC software”
Flashing it with an older TWRP (twrp-3.6.2_9-0-herolte.img) allowed the device to boot normally again.
But when booting into recovery, TWRP is unable to decrypt the device …
My advise was decrypting the device in e-0.20-o before booting in TWRP to backup it and install e-1.4-q
or
Use the recovery-e to install e-1.4-q
I did decrypt in twrp and took a backup of /data. Then I installed e-1.4.q. This overwrote recovery with e-recovery. Then encrypted in e-1.4-q. Then I flashed twrp back, but cannot restore the backup because twrp cannot decrypt /data. (But /e/ is able to decrypt it and boots normally.)
So maybe twrp already needs to be there when the encryption is added?
When one searches the forum for encryption solid answers are difficult to find especially for Samsung.
You are writing about TWRP + decrypt.
Do you want the phone encrypted? This is set at OS level. In Android Q, encryption is now the default. If you go to Settings > Security > Encryption and credentials this is where encryption is set / changed.
The phone is encypted. I set that up after the upgrade, but before flashing TWRP onto recovery.
But TWRP also needs to read the encrypted partition, so it prompts for the decryption pattern when starting. This has worked for the last ~2 years with /e/OS 0.x through nougat and oreo and various TWRP versions.
Somebody suggested an even older version of TWRP, I’ll give that a go when I find time.
I don’t think I had an encrypted partition and I did not format data before the upgrade.
I was prepared to lose all the data, though. So, it wasn’t that bad, I just formated data via /e/recovery and started from scratch. I had back-upped almost everything (every app) manually before (not only a twrp backup). Other data was synced with /e/cloud.
I think your /data have to be not encrypted to be backedup in TWRP
And /data have to be not encrypted to be abble to restore the backup …
I agree.
And I repeat that that is exactly what had been working before and is no longer working now (decrypting /data in TWRP). This might or might not be related to /e/, but is quite definitely a TWRP issue.