This issue affects Adobe Photoshop and the Dropbox backup features on Mac ****** Sur.
When the Desktop and Documents are selected as Personal backup folders, it seems to mess up the Adobe permissions, so that the Save for Web option in Photoshop doesn't work. Only when you deselect the Desktop and Documents backup options do the permissions reset.
Not sure if this is a Dropbox problem or Adobe problem. I found the solution in the Adobe forums here: https://community.adobe.com/t5/photoshop-ecosystem-discussions/p-save-for-web-error-a-write-permissions-error-has-occurred-photoshop-22-1-1/m-p/11747734
Would be good if we could fix this so I can continue to back up my Desktop and Documents folders.