Problem SOLVED
The grandchildren haven't arrived yet, so I made a backup on one of the 3 uSD cards, then booted from yesterday's uSD card backup.
Update, then I downloaded the upgradable list, which I wanted to copy into a text file. Then the icons disappeared and I couldn't do the copying.
I turned it off and restarted from the usb3.1 stick. Update again, get a list, then upgrade and copy the operation and the list to a text file.
Now the DSI screen does not flash after the upgrade.
The error was caused by the uSD card used as a backup.
Fortunately, I mark the uSD cards with colors so that I don't mix them up, so it can be determined that this card is indeed faulty.
I ask for everyone's forgiveness and understanding.
I hope the followers learn from this too.
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Gyula HA3HZ
HA3HZ