I cannot see customer's screen. Black bar on customer side is stuck on connecting. What should I do?

One of the reason for this can be Anti-Virus installed on customer's machine. If that AV is not white-listed then I blocks Support.com Cloud's app. Please follow these steps-

1. Go to AV and disable it temporarily 
2. Run Connect file again
3. Note down AV name and version
4. Share it with us at help@support.com with session id
5. We'll whitelist that AV for to avoid such issues in future