This usually happens when loading external SWF files that use a different version of the classes that are in the main application.
This may be due to the fact that you compiled some RSL modules, modules, or other external SWFs for another version of the Flex SDK. It may also be that you changed the class and did not recompile all the SWFs again.
Sometimes you can do it right, but the old SWF is cached by your browser and therefore is incompatible with your main application. You can work around this by adding a version number to the file names so that the old cached version is never used.
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