The development builds after 2020.12 doesn't work for PW1. It restructures the cache, reload all cover thumbnails, then I get an error message "No reader engine for this file or invalid file" whichever book I want to open. The only solution is downgrade to the latest stable, however, no cover is shown after that.
Try with a nightly, there was a memory alignment issue in the zstd version used in the release, that was hotfixed a few hours later. It *would* unfortunately be fatal on a Kindle, given how the alignment trap is setup there .

(I checked on a K3 a few days ago, and everything behaved as it was supposed to there).
The problem first appeared with nightly 24, and it is the same even now with 30. I have a modern interface with no reader engine it seems.
PW1 is flat, so, more on that later, but 2020.12-27 worked out just fine on a K3, and -30 just fine on a PW2.
Thanks, I am looking forward to it. In the meantime I deleted my KOreader instance then copied the latest nightly to my reader. It was all the same :-(
Well, I did find a fun Kindle special bug in the process, but everything's also fine on my end on a PW1 on 5.6.1.1 with -30.

I would have said try a clean install, but apparently you did, so, check the crash.log instead (in koreader/crash.log).
12/23/20-16:32:30 INFO opening file /mnt/us/koreader/crash.log
[!] doShowReader coroutine crashed:
frontend/apps/reader/readerui.lua:226: attempt to call method 'isStub' (a nil value)
stack traceback:
frontend/ui/widget/widget.lua:48: in function 'new'
frontend/apps/reader/readerui.lua:578: in function 'doShowReader'
frontend/apps/reader/readerui.lua:536: in function <frontend/apps/reader/readerui.lua:535>12/23/20-16:32:30 INFO FM loaded plugin SSH at plugins/SSH.koplugin
I'm going to need the full context, not just an excerpt.
I hope it is ok for you renamed to txt and attached.
Which FW version is your PW1 running on?