dear all
i do have a workflow that sends email to the new owner whenever a case is reasigned, it was working fine, now it is not working.
please check below.

Plugin Trace:
[Microsoft.Xrm.Sdk.Workflow: Microsoft.Xrm.Sdk.Workflow.Activities.SendEmail]
[SendEmailStep1: send email]
Error Message:
Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=9.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Invalid Party Object Type 9Detail:
<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">
<ActivityId>c2a77d06-6a1e-494b-aff5-0dfb1419b828</ActivityId>
<ErrorCode>-2147207930</ErrorCode>
<ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic">
<KeyValuePairOfstringanyType>
<d2p1:key>ApiExceptionSourceKey</d2p1:key>
<d2p1:value xmlns:d4p1="www.w3.org/.../XMLSchema" i:type="d4p1:string">Plugin/Microsoft.Crm.Common.ObjectModel.EmailService</d2p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d2p1:key>ApiOriginalExceptionKey</d2p1:key>
<d2p1:value xmlns:d4p1="www.w3.org/.../XMLSchema" i:type="d4p1:string">Microsoft.Crm.CrmException: Invalid Party Object Type 9 ---> Microsoft.Crm.CrmException: Invalid Party Object Type 9
at Microsoft.Crm.Common.ObjectModel.ActivityPartyService.Create(IBusinessEntity entity, ExecutionContext context)
at Microsoft.Crm.ObjectModel.CommunicationActivityServiceBase.CreateCommunicationPartiesInternal(BusinessEntityMoniker moniker, CommunicationActivity activity, ExecutionContext context)
at Microsoft.Crm.BusinessEntities.BusinessProcessObject.PostCreateInPipelineEventHandler.Invoke(Object sender, ExtensionEventArgs e)
at Microsoft.Crm.BusinessEntities.BusinessProcessObject.DoCreate(IBusinessEntity entity, ExecutionContext context)
--- End of inner exception stack trace ---
at Microsoft.Crm.Extensibility.VersionedPluginProxyStepBase.Execute(PipelineExecutionContext context)
at Microsoft.Crm.Extensibility.PipelineInstrumentationHelper.Execute(Boolean instrumentationEnabled, String stopwatchName, ExecuteWithInstrumentation action, PipelineExecutionContext context)
at Microsoft.Crm.Extensibility.Pipeline.<>c__DisplayClass2_1.<Execute>b__0()</d2p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d2p1:key>ApiStepKey</d2p1:key>
<d2p1:value xmlns:d4p1="schemas.microsoft.com/.../Serialization" i:type="d4p1:guid">8c3b8615-ecd8-db11-b397-0019b9204da9</d2p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d2p1:key>ApiDepthKey</d2p1:key>
<d2p1:value xmlns:d4p1="www.w3.org/.../XMLSchema" i:type="d4p1:int">5</d2p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d2p1:key>ApiActivityIdKey</d2p1:key>
<d2p1:value xmlns:d4p1="schemas.microsoft.com/.../Serialization" i:type="d4p1:guid">c2a77d06-6a1e-494b-aff5-0dfb1419b828</d2p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d2p1:key>ApiPluginSolutionNameKey</d2p1:key>
<d2p1:value xmlns:d4p1="www.w3.org/.../XMLSchema" i:type="d4p1:string">System</d2p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d2p1:key>ApiStepSolutionNameKey</d2p1:key>
<d2p1:value xmlns:d4p1="www.w3.org/.../XMLSchema" i:type="d4p1:string">System</d2p1:value>
</KeyValuePairOfstringanyType>
</ErrorDetails>
<Message>Invalid Party Object Type 9</Message>
<Timestamp>2018-11-06T11:14:24.3114443Z</Timestamp>
<ExceptionRetriable>false</ExceptionRetriable>
<ExceptionSource i:nil="true" />
<InnerFault>
<ActivityId>c2a77d06-6a1e-494b-aff5-0dfb1419b828</ActivityId>
<ErrorCode>-2147207930</ErrorCode>
<ErrorDetails xmlns:d3p1="schemas.datacontract.org/.../System.Collections.Generic">
<KeyValuePairOfstringanyType>
<d3p1:key>ApiExceptionSourceKey</d3p1:key>
<d3p1:value xmlns:d5p1="www.w3.org/.../XMLSchema" i:type="d5p1:string">Plugin/Microsoft.Crm.Common.ObjectModel.EmailService</d3p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d3p1:key>ApiOriginalExceptionKey</d3p1:key>
<d3p1:value xmlns:d5p1="www.w3.org/.../XMLSchema" i:type="d5p1:string">Microsoft.Crm.CrmException: Invalid Party Object Type 9 ---> Microsoft.Crm.CrmException: Invalid Party Object Type 9
at Microsoft.Crm.Common.ObjectModel.ActivityPartyService.Create(IBusinessEntity entity, ExecutionContext context)
at Microsoft.Crm.ObjectModel.CommunicationActivityServiceBase.CreateCommunicationPartiesInternal(BusinessEntityMoniker moniker, CommunicationActivity activity, ExecutionContext context)
at Microsoft.Crm.BusinessEntities.BusinessProcessObject.PostCreateInPipelineEventHandler.Invoke(Object sender, ExtensionEventArgs e)
at Microsoft.Crm.BusinessEntities.BusinessProcessObject.DoCreate(IBusinessEntity entity, ExecutionContext context)
--- End of inner exception stack trace ---
at Microsoft.Crm.Extensibility.VersionedPluginProxyStepBase.Execute(PipelineExecutionContext context)
at Microsoft.Crm.Extensibility.PipelineInstrumentationHelper.Execute(Boolean instrumentationEnabled, String stopwatchName, ExecuteWithInstrumentation action, PipelineExecutionContext context)
at Microsoft.Crm.Extensibility.Pipeline.<>c__DisplayClass2_1.<Execute>b__0()</d3p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d3p1:key>ApiStepKey</d3p1:key>
<d3p1:value xmlns:d5p1="schemas.microsoft.com/.../Serialization" i:type="d5p1:guid">8c3b8615-ecd8-db11-b397-0019b9204da9</d3p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d3p1:key>ApiDepthKey</d3p1:key>
<d3p1:value xmlns:d5p1="www.w3.org/.../XMLSchema" i:type="d5p1:int">5</d3p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d3p1:key>ApiActivityIdKey</d3p1:key>
<d3p1:value xmlns:d5p1="schemas.microsoft.com/.../Serialization" i:type="d5p1:guid">c2a77d06-6a1e-494b-aff5-0dfb1419b828</d3p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d3p1:key>ApiPluginSolutionNameKey</d3p1:key>
<d3p1:value xmlns:d5p1="www.w3.org/.../XMLSchema" i:type="d5p1:string">System</d3p1:value>
</KeyValuePairOfstringanyType>
<KeyValuePairOfstringanyType>
<d3p1:key>ApiStepSolutionNameKey</d3p1:key>
<d3p1:value xmlns:d5p1="www.w3.org/.../XMLSchema" i:type="d5p1:string">System</d3p1:value>
</KeyValuePairOfstringanyType>
</ErrorDetails>
<Message>Invalid Party Object Type 9</Message>
<Timestamp>2018-11-06T11:14:24.3114443Z</Timestamp>
<ExceptionRetriable>false</ExceptionRetriable>
<ExceptionSource i:nil="true" />
<InnerFault>
<ActivityId>c2a77d06-6a1e-494b-aff5-0dfb1419b828</ActivityId>
<ErrorCode>-2147207930</ErrorCode>
<ErrorDetails xmlns:d4p1="schemas.datacontract.org/.../System.Collections.Generic" />
<Message>Invalid Party Object Type 9</Message>
<Timestamp>2018-11-06T11:14:24.3114443Z</Timestamp>
<ExceptionRetriable>false</ExceptionRetriable>
<ExceptionSource i:nil="true" />
<InnerFault i:nil="true" />
<OriginalException i:nil="true" />
<TraceText i:nil="true" />
</InnerFault>
<OriginalException i:nil="true" />
<TraceText i:nil="true" />
</InnerFault>
<OriginalException i:nil="true" />
<TraceText>[Microsoft.Xrm.Sdk.Workflow: Microsoft.Xrm.Sdk.Workflow.Activities.SendEmail]
[SendEmailStep1: send email]
</TraceText>
</OrganizationServiceFault>
at Microsoft.Crm.Workflow.Services.SendEmailActivityService.Execute(ActivityContext executionContext, SendEmail sendEmail)
at System.Activities.CodeActivity.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation)