After the upgrade to 21.7 it takes Dreamweaver endless time to connect a file on the iCloud, the wheel of hell rotates for minutes now. Any idea to fix it?

iCloud is a cloud drive. 

For best results, use cloud drives for backups only. 

Move local site folders to your primary HD, the same as Dreamweaver. You’ll enjoy faster performance and more stable file read/write operations.

If you’ve already tried the typical troubleshooting steps like Restoring Preferences (even on a fresh install, things can be corrupt there ) or uninstall/reinstalling the program through the CC Desktop App to no avail, I’d probably roll back to the older version, at least then you can continue working.