Since it might not be fix-able with the nvidia GPU & drivers (and this might not work while I have other acceleration-requiring apps I need to share), I've temporarily followed Zoom's directions here to only use the Integrated Graphics.   Unclear what kind of new problems this will introduce, but I'll post those here to help other users who may be searching for this in the future. Black screen during screen sharing – Zoom Support  
Hello; This problem has plagued this Dell Precision 7550 laptop for at least one year.  I always keep drivers, BIOS, and packages updated monthly, and I even install SupportAssist during these monthly updates to help ensure accuracy of driver selections.  (I then uninstall SupportAssist to prevent it from chewing up cycles without prompting, which it does regularly if you keep it installed -- that's a different issue.) All nvidia and Win10 driver settings are the default, with only two exceptions: - I keep brightness all the way down, because these screens are far too bright (to an unhealthy degree). - I keep Win10's "Night Light" setting enabled with another 30% dim, because these screens are far too bright (to an unhealthy degree).   The problem is this:  After most of the driver updates in the last ~year, even after an otherwise clean working session for days, as soon as I click "Share Screen" in Zoom or WebEx, the entire screen repeatedly flickers black / on slowly, as if it's trying to resync/detect the resolution and color palette, and ultimately the screen sharing will fail.  Audio remains fine the whole time.  Sometimes a 2nd or 3rd attempt to share will work, sometimes I simply cannot present. At this point, it's actually preventing me from doing my job on a regular basis. This does NOT happen on MS Teams, which (if I'm not mistaken) uses a form of their Remote Desktop Protocol (which also works normally on this laptop).  Something about Zoom and WebEx conflicts with the video card or drivers in this 7550.   This is likely unrelated, but I also have Dell Optimizer installed, but only to enable the physical mic/headphone jack -- everything else is disabled in that app, and I never need to open it. There is never an external monitor in any scenario.   Anyone else encountered this?
Precision 7550 screen blacks out and flickers during screen sharing in Zoom or 
WebEx