Well, I’ve found the detritus causing your problem. You can’t find its origin.
I can’t help any further…
I’m sorry too.
But once deleted the detrius file, and the error still exits, what can I do?
If you remove the detritus, no detritus error should exist.
If .DS_Store is the problem then you need to find where are normally the other files (csv, png, rtf) that end on the same folder, then remove .DS_Store there so it doesn’t end here.
Did you try the input from the other thread already?
It did seem to work for Michel (slightly different occurrence from yours, but it doesn’t hurt to try).
I did experiment with Permissions Reset copying to a DMG and back again, but the com.apple.macl was still there.
Zip certainly is a possibility, but I am concerned that Apple is adding or preserving com.apple.macl when its done via an application. If my theory is true, then a 3rd Party application can no longer assist in these matters 
I think you need a some specific kind of volume to avoid preserving it. FAT maybe? I also remember something about removing ownership of the volumes. Certainly APFS must be avoided.
Also do not copy there and back. Copy there, delete the origin, and copy back.
Maybe zip does it faster.
Thank’s Alberto I found (?) a solution:
I created a new clean folder and child folders and copy and paste (not move) every file from the old project folder and,… mystery, the error disappeared.
Don’t ask me why
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Than’s Rick
Enric:
as far as I know, macOS creates a .DS_Store when you open a folder.
Before that, the folder is empty.
[OFF] @Sam, others
With 11.5.2, I saw a “new file” appearing: Icon?. I made a copy of a folder with custom Icons (yes, custop icons pasted on macOS folder default) from Big Sur volume to intel created HDD. That file appears in the HDD copied folder.
Then it’s probably a (hidden) file that’s not allowed to be codesigned.
Wasn’t it explained before in this thread that your .DS_Store files were taken from the source folder so you needed to remove them there?
Such a file has existed from ages in Mac OS, with little variation (it was once known as “Icon
” (yes, an end of line)). This is the file that holds the icon for folders having a custom icon (the file is stored right into the given folder). Nothing new.
“Excepted variation”, I know.
Replacing on external HDD \Return with a question mark is new and clearly a bug: macOS do not found the asssociated icon file, so it does not display it…
You mean the icon file is incorrectly created with the name “Icon?” (which, in turn, makes Mac OS not showing the custom icon) and if you manually replace “?” by an end of line, it gets fixed?
Since which Mac OS release have you seen this bug? I’m still on 10.14, but my last custom icon for a folder is now dating.
(and I’ve bought a Mac mini which should come soon, to test Big Sur and later systems; can’t wait 
)
If you followed the orders exactly as requested, we could have ended faster. It was there.
ls does not show hidden files.
To see all files, including hidden ones, you must cast: ls -a