Hi Sanele!
What is your current environment? On-premise or Online?
Does anyone else have the same issue in your organization?
What is your identity when you join the meeting using the web app? Guest or Skype for Business user account?
Are there any error messages when join the meeting with Skype for Business App?
Considering this issue is only occurred in Skype for Business, it recommends you make sure Skype for Business client is up to date and clear any cached credentials from previous logons.
Besides, you can also try to run the Microsoft Remote Connectivity Analyzer(The link is: https://www.testconnectivity.microsoft.com/tests/skype)
to see if there is any error message.
For more details about troubleshooting this issue, you can refer to:
https://docs.microsoft.com/en-us/skypeforbusiness/troubleshoot/online-meetings/cannot-join-online-meetings#troubleshooting-windows-client-issues
Best Regards,
Jimmy Yang
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