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After upgrading a user to Notes 6.5.4, she got an error saying “Illegal Circular USE:DocumentConversions” when opening her mailfile. She reported that she’d get this problem when opening folders.
The key to solving this problem, is knowing that the user has been upgraded from Notes version 5 to Notes version 6.5.4. From one version of Notes to the next, new functionality is implemented using design changes to the mailfile. When a user upgrades from one version to the next, the design need to be upgraded as well.
If, after replacing the design, the problem still occurs, you should try refreshing it as well. This has solved the problem for me a couple of times.