That's a tough one, it seems that there might be a corrupted record int eh database, causing the service valudation to fail when retrieving the items. As this is a public post, maybe it would be more convenient if you open a support ticket with us where we can arrange you providing us with temporary backend login credentials, or maybe the site files and DB backup so we cna debug the issue locally.
In the meantime, just to gather more information about the case, can you please check the App_Data/Sitefinity/Logs folder - this is the location where Sitefinity keeps its error log. The best way to proceed is move(or remove) all the contents of this folder, and then reproduce the issue - Sitefintiy will generate a fresh log file containing the registered exception, if any. Please provide us with this log file, so we can observe it for any additional information that could help us proceed in resolving the case.
Thnaks in advance for your cooperation.
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