Whilst on customer site I experienced a strange issue with System Center Operations Manager 2012. I had cleared down some alerts inside the console, however one would not disappear. Each time I tried to remove the alert the following error message would show:-
"An object of type MonitoringAlert with id xxx was not found.
I tried closing and reopening the console and even restarted the server all to no avail.
Solution:- It turns out that the console content is stored in an MDB file, and this can, in rare instances become corrupted.
To be able to reset the status of the console it's best to add a
/clearcache option on to your SCOM Console
More info can be found here - http://bit.ly/1k35dzM
Hope this helps!