Some third party CS5 plugins were able to function in CS5.5. This was unintended behavior and that was removed in the CS5.5 7.5.1 update. If you rely on third party plugins which are not yet updated, it might be advisable to wait to update until your third party plugins are updated as well. For more information see:
http://kb2.adobe.com/cps/908/cpsid_90828.html
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