Resolve case does not work

Hi All,

I have a problem in Dynamics 365 V9.1.0.876., when i try to close case, i get this error:

Element 'schemas.microsoft.com/.../Arrays:anyType' contains data from a type that maps to the name 'Microsoft.Crm.Common.ObjectModel:ActivityState'. The deserializer has no knowledge of any type that maps to this name. Consider changing the implementation of the ResolveName method on your DataContractResolver to return a non-null value for name 'ActivityState' and namespace 'Microsoft.Crm.Common.ObjectModel'.