I'm getting this error when I try to run Get/New-InboxRule on a specific mailbox on Exchange 2010 SP2:
A participant ENTRYID is malformed and cannot be read.
+ CategoryInfo : NotSpecified: (0:Int32) [Get-InboxRule], CorruptDataException
+ FullyQualifiedErrorId : 129AF165,Microsoft.Exchange.Management.RecipientTasks.GetInboxRule
Trying to create the rule in OWA gets the same result.
It turns out the error was the result of a invalid rule (the destination folder didn't exist). After I deleted that rule through Outlook, everything worked fine. I'm surprised that I couldn't get any results on Google for "A participant ENTRYID is malformed and cannot be read."