As of recently I get a server error when I try to create a new folder in the media manager.

"Unable to load /EPiServer/cms/Stores/contentdata/ status: 500",

I already had a look at this topic without any luck http://world.episerver.com/Modules/Forum/Pages/Thread.aspx?id=85177. 

This is the callstack:

System.ArgumentException: An item with the same key has already been added.
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at EPiServer.Cms.Shell.Json.ContentDataStoreModelConverter.GetContentType(String typeIdentifier)
   at EPiServer.Cms.Shell.Json.ContentDataStoreModelConverter.ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at EPiServer.Shell.Services.Rest.RestControllerActionInvoker.TryGetParameterValueFromRestProviders(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor, Object& value)
   at EPiServer.Shell.Services.Rest.RestControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor)
   at System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass21.b__19(AsyncCallback asyncCallback, Object asyncState)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout)
   at System.Web.Mvc.Async.AsyncResultWrapper.Begin[TResult](AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate`1 endDelegate, Object tag, Int32 timeout)
   at System.Web.Mvc.Async.Asy..