Error in edit view - equence contains more than one element

We started to see this kind of error messages in the edit mode of a QuickSilverbased CMS/Commerce site running version CMS 9.8.2 and Commerce 9.13. No real idea of why it has occurred.
317 {message: "Unable to load /episerver/cms/Stores/contenttype/?dojo.preventCache=1463574636969 status: 500", response: Object, status: 500, responseText: "
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↵        ", xhr: XMLHttpRequest}
message
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"Unable to load /episerver/cms/Stores/contenttype/?dojo.preventCache=1463574636969 status: 500"
response
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Object
responseText
:
"
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↵        Sequence contains more than one element
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↵            Server Error in '/' Application.
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↵             Sequence contains more than one element 
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↵             Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
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↵             Exception Details: System.InvalidOperationException: Sequence contains more than one element


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↵            Source Error: 


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↵An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
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↵            Stack Trace: 


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↵[InvalidOperationException: Sequence contains more than one element]
↵   System.Linq.Enumerable.SingleOrDefault(IEnumerable`1 source) +423
↵   EPiServer.DataAbstraction.RuntimeModel.ModelAttributeCollection.TryGetSingleAttribute(T& attribute) +59
↵   EPiServer.Cms.Shell.UI.Rest.ContentTypeStore.GetImagePath(ContentType type) +131
↵   EPiServer.Cms.Shell.UI.Rest.ContentTypeStore.ToContentTypeViewModel(ContentType type) +370
↵   System.Linq.WhereSelectListIterator`2.MoveNext() +111
↵   Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) +549
↵   Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType) +409
↵   Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType) +1341
↵   EPiServer.Framework.Serialization.Json.JsonObjectSerializer.Serialize(TextWriter textWriter, Object value) +71
↵   EPiServer.Shell.Services.Rest.RestResultBase.ExecuteResult(ControllerContext context) +298
↵   EPiServer.Shell.Services.Rest.RestResult.ExecuteResult(ControllerContext context) +23
↵   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +90
↵   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +833
↵   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +833
↵   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +81
↵   System.Web.Mvc.Async.<>c__DisplayClass21.b__1e(IAsyncResult asyncResult) +186
↵   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +38
↵   EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult) +60
↵   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +399
↵   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +137
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↵            Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.1073.0
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