When screen sharing people see black screen

I had similar issue - difference in my case was that the problem of black screen share and stuck video, was a problem on wired ethernet connection only. Wi-Fi was fine but I as a network engineer I wanted to understand the problem.

Anyhow, I still don’t know what the root cause although it appears QoS related.

The fix that worked for me:

- on your router block your LAN traffic ( source) going to internet on port 3479,3480 and 3481 destination
- in other words - block Lan Traffic source ; destination internet and destination ports: 3479,3480 and 3481.

I believe this forces mSTeams traffic to go via port 443 which is what the web version uses.

I hope it works for you.

As for a suggestion of a mate of mine, I entered a registry key to disable QoS all together for Teams. My understanding has been that you have to deploy a GPO to enable QoS for Teams as per following article: https://docs.microsoft.com/en-us/microsoftteams/qos-in-teams-clients. But it looks like that Teams desktop client is getting its QoS policy from the server, by the setting "Meetings\Meetings Setting\Network\Insert Quality of Service (QoS) markers for real-time traffic". This is my opinion, since I have not found any information to back it up.
How I fix it:
1. I created the registry key "HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\QoS" and the following registry entries under it (this entries are in the article but with the QoS values):
Name: Application Name
Type: REG_SZ
Data: Teams.exe

Name: DSCP Value
Type: REG_SZ
Data: 0 (This is Zero to disable QoS)

Name: Local IP
Type: REG_SZ
Data: *

You need to reboot.
Apparently, Microsoft is coming up with a fix in August.