No input response. No terminal available. Have to switch off power. Which log file do I look at after reboot?
Connect from another system via USB serial and minicom to see what’s going on… I use a “Prolific Technology, Inc. PL2303 Serial Port / Mobile Action MA-8910P” device to connect to my RPi’s.
If you have another computer and the Pi has sshd running - you might be able to ssh into the Pi to see what is going on.
I had to do that with Ubuntu at the begining of Pi4b. I switched to OpenSUSE 15.3 on my Pi4’s and had no lockups. I installed ext4 file with MATE Desktop.
Thanks to both of you for your suggestions, but I think they are slightly out of my range of abilities. The odd observation is that the lockups only started since the last two software updates. I have not installed anything new. So I will work with another O.S. and see if the problem disappears after a few more of the frequent updates.