"VerifyError: Error #1053: Illegal Override of CustomProxy...", for plugin in Web Client 6.0

I am having an issue with a plugin developed for 5.1. Works fine with both web client 5.1 and 5.5.

I am trying to deploy the same plugin in web client 6.0 and made following changes with reference of this link for compatibility:

2 Important reminders for your 5.5 plugin to be compatible with Web Client 6.0

-  Changed Web-ContextPath, vsphere-client/plugin-name in MANIFEST.MF

-  Configured two default channels, amf and secure-amf, in services-config.xml and remote-config.xml. :

After these changes, I was able to resolve the "Unable to load resource module..." errors. And I am able to see the icon of my plugin.

I made changes to my own proxy(CustomProxy) class to use ServiceUtil.getDefaultChannelUri(). I have added ServiceUtil code to my plugin instead upgrading from SDK 5.1 to SDK 5.5.2.

Now getting following error upon clicking the plugin icon:

"VerifyError: Error #1053: Illegal Override of CustomProxy...". Please find the screenshot of the flash player error attached.

Could somebody guide to possible cause and resolution for the issue?

Thanks.