I am not sure when this began happening, but as of this week, when attempting to access a web ex meeting, our users are prompted repeatedly for proxy credentials. It is not the native OS X proxy prompt, but a generic window titles 'Proxy Authentication Required'. Entering the user name and password to authenticate does not work, and the prompt reappears. I am seeing similar issues with flash content, and also repeated prompts related to java - the issues are not specific to Safari but also affect Firefox as well.
I exported the keychain items related to the proxy credentials for http and https into the system keychain, but this did not resolve the issue.