Please check the following:

1. This problem may be caused by a clash with another meeting being held. Please end the meeting.
    
2. A meeting held before a recurring meeting may not have finished yet.
   Refer to the following FAQ:
   FAQ : I cannot get into the online meeting room when holding recurring meetings.
    
3. Check if your computer is connected to the Internet.
    
4. Verify you are using the correct Account name and password (Host only).
   Refer to the following FAQ:
   FAQ : I cannot get into the online meeting even if I enter the correct password.The message "Password required" is displayed.
      
5. If the Host locked the meeting room, you cannot join the meeting until the meeting room is unlocked.
   Please confirm the meeting room is unlocked.
      
6. Make sure you allow access to OmniJoin if Windows Firewall prompts you to block the application.
   If you have chosen to block the application, you need to change the settings.
    
7. If you have a personal firewall, be sure to allow OmniJoin to access the Internet (ports 443 and 80).
   OmniJoin may be restricted by a security policy on your network.
   Contact your network administrator about Firewall, NAT or router settings.
    
8. A system error may have occurred. Restart OmniJoin.
    
9. Restart your computer.
    
10. The server may be temporarily unavailable, or the network may be congested.
    Please wait for about 10 minutes and try again.
     
11. Create and hold a new Meeting.
     
12. The invited meeting version and the version you are using may be different.

    Refer to the following FAQ:
    FAQ : The error message "Unknown join error." or "The meeting you specified does not exist or has already ended." is displayed.