About the broken “Chernobyl” update lineage-17.1-20210426-nightly-FP3-signed.zip:
I examined it in detail, found four missing files, namely
vendor/lib/libbson.so
vendor/lib64/libbson.so
vendor/lib/libqsap_sdk.so
vendor/lib64/libqsap_sdk.so
as well as other inconsistencies on the vendor partitions, such as an outdated build fingerprint in vendor/build.prop. I can only assume that the LineageOS build system produced a spuriously broken build.
For better debugging of possible future boot failures I have activated the kernel ramoops feature. My advice for all users is: don’t immediately install the next update on coming Monday, let us wait and I will check if things are in working order again.
Another thing: if an update like this breaks again, there is no need to use manual fastboot commands to change slots back to the last working state, especially if you are on the run and can’t access another machine with adb or fastboot commands. When you land in the LineageOS recovery menu saying Can't load Android system at the top, simply press the menu option Try again. Do this several times. The bootloader logic will automatically switch slots after several attempts, and you will end up on the other slot with LineageOS booted up again normally.