Are you using the Webex desktop client or the Webex browser client [thin client] ?
Can you join other Webex sites successfully? This appears to be a client related crash of the Webex meeting client and normally clears up on a reboot [which of course you've tried].
To troubleshoot this effectively could you please contact our TAC organization and open a case?
Possible workarounds: Try a different browser, Use the Webex Desktop app and enter the meeting iD/personal room/webex invite URL you are trying to join.
It's possible an intermediary is resetting the connection [VPN, Proxy, application layer gateway], and you may try bypassing those as well.
It is also possible this is a certificate exchange/TLS handshake problem that TAC is best able to debug, but can occur for instance if a TLS proxy is inline or the system date/time is out of date.
Thank you
support.cisco.com, also 800-553-2447
