An odd issue I encountered today at one of my customers that has recently completed a migration to SCOM 2012 R2, when trying to open the overrides node with no scope selected I got the following error “An object of class ManagementPackClass with ID <GUID Removed> was not found” and then the view shows 0 overrides until the console is reopened with a /clearcache.
Turns out this error is caused by having an override that references a class that no longer exists, in order to track it down you first have to access the overrides which you can no longer see unless you do the following:
1. Export your overrides to text file using poweshell by running get-scomoverride | out-file c:\SCOMOverrides.txt
2. Search the output text file for the GUID which was in the original error “An object of class ManagementPackClass with ID <GUID Removed> was not found”
3. The GUID will appear on a line labeled Context, below that one look for a line labeled Identifier, In my case it looked as follows “Identifier: 1|Windows.Operating.System.Custom.Monitors”
4. Generally this is enough information to identify which management pack needs to be deleted if it is not then continue to step 5
5. In SQL run the following query, remebering to edit it for your Identifier, against your operationsmanager database select * from ManagementPack where ManagementPackSystemName like ‘Windows.Operating.System.Custom.Monitors’
6. Once you have identified which management pack is causing your issue it needs to be deleted, once the new configuration is processed you will be able to see your overrides again.
Solution provided by Mcirosoft on Technet forum