Bug: Black Dialogue Window for Save For Web (Legacy)
OS: Mac OSX BigSur 11.1
Photoshop Version: v20.01 (also had similar issue in v19)
I updated to version 20.01 in hopes to resolve this issue I have had in previous versions. This reason motivated me to updated photoshop shop to see if this UI bug was fixed. When opening the export dialogue from Export > Save For Web (Legacy) window, it shows up black. I can see the options but the preview and text are not present.
We're sorry for the Save for web UI issue. Have you tried updating Photoshop to the 22.4.2 version and check if that helps to resolve the UI issue? I have just tested it on Photoshop 22.4.2 and macOS 11.4 version and it's working fine.
Also, turn off the graphics processor from the Photoshop preferences > Performance tab > Restart Photoshop and see if that helps.