The KB3069392 update from 14 July 2015 prevents WebEx screen sharing by computers using Windows. Viewers of the sharing are fine, even on Windows.
The KB3069392 update is described at https://support.microsoft.com/en-us/kb/3069392
WebEx says that the bulk of their calls today are due to this problem. They say that uninstalling this update fixes the problem, but WebEx doesn’t recommend removing Microsoft security updates.
WebEx claims that the problem can also be fixed by changing the Windows Personalization theme to Basic, as described by going to https://support.webex.com/MyAccountWeb/knowledgeBase.do?root=Tools&parent=Knowledge and searching for WBX30174 (there doesn’t seem to be a direct URL). However, this didn’t work for me, even after they had me try uninstalling and reinstalling the WebEx software and wiping out all my browser cookies.
The screen sharing works in some strange combinations of closing and restarting the meeting, but I haven't found anything dependable as a workaround.
This appears to affect all versions of Windows, but due to the design of Microsoft Communities I'm forced to choose one version of Windows for this report.
If anyone knows of a workaround, please post it.
If an update is posted, whoever finds out should give the rest of us a heads-up.