twrp – Device stuck in boot animation after reverting to LineageOS 17.1

I tried to update my OnePlus One to LineageOS 18.1, following my usual procedure:

  • do a full backup (boot, system, data excl. storage, cache) with TWRP
  • then flash the new image, along with the zip files Magisk and microG

As I then noticed that Android 11 breaks some of the apps I use, I decided to roll back the update by simply booting into recovery, restoring the backup and rebooting. However, this failed with the system being stuck in the boot animation for significantly above 10 minutes.

I then tried the following:

  • boot into TWRP
  • flash LineageOS 17.1 from the previous image I had installed (plus Magisk and microG)
  • restore my backup
  • wipe caches

Still no joy, I am still stuck in the boot animation.

Suspecting the upgrade had converted some app data in /sdcard/Android/data, I moved all app dirs starting with com.android or org.lineageos off the device. (The rest should be user app data. I do not have Google Play services installed.)

That still was not successful, I am still stuck in the startup sequence.

I have never (knowingly) encrypted any data on the device.

What gives? How can I find out where the device gets stuck, so I can hunt the error down? Wiping all user data is not an option.