Thanks for your help.
For Dropbox I followed the instructions : logout of my account and delete dropbox app.
After that I did not have a Desktop folder any more, the link remained in my $HOME with no rights to modify.
I have no antivirus.
I also use iCloud and Google drive, but I logged out before doing the Dropbox deletion.
Fix Permission is an option in the Account preferences of the Dropbox app, accessible with the Alt Key + click
The other commands I tried :
sudo chmod -N Desktop
sudo chmod ugo+rw Desktop
sudo chflags nouchg Desktop
sudo xattr -d com.dropbox.attrs Desktop
I also disables SIP in recovery mode.
But I could not change the uchg flag.
Final option : I activated root user
Root deleted the uchg flag
Even the root user cannot change xattr or the owner of the file.
For the moment, I just reactivated Dropbox which created the Desktop folder again so that the link works again.
Desktop -> /Users/alex/Dropbox/My Mac (Alexs-MacBook-Air.local)/Desktop
Here is the result of ls :
ls -hleO $HOME
total 16
drwx------@ 5 alex staff - 160B Apr 18 2020 Applications
ls: /Users/alex/Desktop: Permission denied
lrwx------ 1 mac staff - 60B May 11 2021 Desktop
drwx------+ 16 alex staff - 512B May 14 16:57 Documents
0: group:everyone deny delete
drwx------@ 9 alex staff - 288B May 18 19:52 Downloads
0: group:everyone deny delete
1: group:com.apple.sharepoint.group.4 allow search
drwx------@ 13 alex staff - 416B May 14 18:23 Dropbox
and xattr
xattr Desktop
com.dropbox.attrs
