FIXED: SEE UPDATE AT BOTTOM
I upgraded one of my computers to Windows 10 and ran into a problem with WebEx Meeting Center and Event Center. Attempting to join a meeting in progress resulted in the following error:
Invalid input or system error. Please try again or contact your site administrator.
A quick search on Google revealed several threads dedicated to this topic. Users reported the error with all the major browsers: Internet Explorer, Edge, Chrome, and Firefox.
From the context of the messages, I get the impression that WebEx worked fine with the initial release version of Windows 10, but an update or system patch in early September seems to have broken the functionality. One user tracked patches back to a potential problem with KB2267602 - a definition update for Windows Defender, but that has not been verified as the culprit by Microsoft or Cisco.
Another user says that WebEx is not properly parsing the Windows 10 User Agent String.
There are workarounds that have been found for Google Chrome (installing a third-party add-in called ieTAB) and Internet Explorer (use Developer Tools to set Document Mode and User Agent String to IE 9), but these are too advanced and intrusive for most casual web conference attendees.
One hopes that Cisco or Microsoft will patch their code soon to resolve the incompatibility, but in the meantime, WebEx meeting hosts should be prepared to field calls from attendees with Windows 10.
UPDATE OCT 1: That was fast! WebEx has fixed the incompatibility with a new Release 30. It lets users join from Windows 10 computers. If you have a hosted account, you should already be upgraded or have it scheduled.
To check your WebEx version number, log in and go to Support - Downloads in the left navigation panel. A page will display with your Meeting Center or Event Center version number. You want to see 30 as the high-level integer portion.
By the way, they altered the user interface slightly, which caused me to spend an hour with a WebEx tech rep, trying to figure out why buttons were not appearing. He didn't know about the changes either. The Participant panel no longer displays buttons at the bottom for Raise Hand and Audio. Raise Hand is now a toggle icon next to the attendee's name, and Audio controls must be accessed from the top command bar. In a nice functionality addition, people with their Chat panel closed now get a pop-up notification if a message comes in.