Device stuck in boot animation after reverting to LineageOS 17.1
Everything that is in the backup is safe to wipe for a test (it can be restored later).
Therefore, rebuild the system step by step:

wipe system, data (excl. storage) and both caches
reinstall LineageOS 17.1
boot
if this works, return to recovery, install Magisk and reboot
if the device still boots successfully, restore data and retry
if the device still boots successfully, copy back app data to /sdcard/Android/data (possibly one by one, or in small batches rather than all at once) and retry
if the device still boots successfully, restore the system partition from backup and retry

If at some point the system no longer boots (i.e. gets stuck in the boot animation or similar), you can examine the step that broke it in more detail.
In my case, it seems the backup of the System partition was faulty and caused the system to hang on boot. Since this partition does not contain any user data, it is easy to rebuild if you still have the system image: Wipe the system partition, reinstall LOS 17.1 and Magisk, wipe cache and Dalvik, then reboot. That fixed it for me.