I have the same problem. When I try to Save for Web in Photoshop I get the error message "Could not complete this operation. An unknown error has occured." I am using version 21.1.1 and Catalina 10.15.4. The bug is still there.
I have hundreds of tiff images that I need to resize and export as jpg, and the 'export as' doesn't get recorded in actions panel. What am I supposed to do? Adjust them one by one?
This is ridiculous. It's an expensive software. This is wrong.