InDesign and InCopy CS5.5 incorrectly allowed CS5 compatible plug-ins to load, leading to code conflicts and instability. This issue has been fixed in the 7.5.1 update for InDesign and InCopy CS5.5. After you install the update, some third-party plug-ins don't load, and produce errors at startup. This issue occurs for plug-ins that have not been updated to be compatible with the new CS5.5 products.
To ensure compatibility and reliability, disable any third-party plug-ins that have not been updated to be compatible with CS5.5. If you want to use the plug-ins at your own risk, avoid installing the 7.5.1 update. Or, if it's already installed, reinstall CS5.5 from your original installer.
It's necessary to update third-party plug-ins designed to work with InDesign CS5 so they work with InDesign CS5.5. Because of a coding error, CS5.5 incorrectly allows these CS5-compatible plug-ins to load. This error was fixed in the 7.5.1 update. Third parties are working on updates to be compatible with InDesign CS5.5. Contact your plug-in authors regarding their schedule for delivering CS5.5 compatible updates.