The Issue:
Instead of Zoom toolbars and floating menus (like the chat window or the participant list), participants see plain gray boxes the same size, shape, and location as the toolbars and floating menus. The cause of this is on the host's computer, but the host does not see the gray boxes - only the participants do. The issue can happen on both Mac and Windows computers with older hardware and operating systems.
Note: Zoom is a dynamic product and is updated regularly to fix issues and provide the best user experience. It is essential to keep your Zoom client application up to date. To update, open the client, click on the upper right corner of the window where the user initials or photo is, and choose "Check for Updates" from the drop down menu.
Causes & Fixes
The most common cause is an out-of-date operating system and/or an out-of-date computer being used by the host. An out-of-date operating system, coupled with the Zoom Sharing setting set to "Optimize for Video Clip," is a primary cause for the gray boxes issue.
FIX #1:
As a host, when sharing your screen in Zoom, uncheck the Optimize for Video Clip option.
FIX #2:
If the above action does not fix the issue, click the More menu [ . . . ] in the Zoom Toolbar and check, "Hide Floating Meeting Controls." You can hit the ESC key on your keyboard to show the menu again.
Additional Resources from the Zoom Help Center
Black Screen During Screen Sharing. Describes a known issue in Apple’s operating systems and blank screens during screen sharing on Windows computers.
MacOS Big Sur black screen when screen sharing.
Optimizing a shared video clip in full screen.