Here is an error I received when attempting to delete a maintenance plan in a SQL 2005 instance using SSMS 2014.
Exception has been thrown by the target of an invocation. (mscorlib)
Additional information:
Index was out of range. Must be
non-negative and less than the size of the collection.
Parameter name: index (mscorlib)
I was able to successfully delete the object by using SSMS 2005.