"Invalid data has been used to update the list item. The field you are trying to update may be read only." (4215000)

When trying to migrate documents, the following error is thrown (this prevents the Rich Text Body field from posting):

Microsoft.SharePoint.Client.ServerException: Invalid data has been used to update the list item. The field you are trying to update may be read only. at Microsoft.SharePoint.Client.ClientRequest.ProcessResponseStream(Stream responseStream) at Microsoft.SharePoint.Client.ClientRequest.ProcessResponse() at Microsoft.SharePoint.Client.ClientRequest.ExecuteQueryToServer(ChunkStringBuilder sb) at Microsoft.SharePoint.Client.ClientRequest.ExecuteQuery() at Microsoft.SharePoint.Client.ClientRuntimeContext.ExecuteQuery() at Microsoft.SharePoint.Client.ClientContext.ExecuteQuery() at Quest.NSP.SharePoint.ClientSPListItem.Update(Boolean preserveAuditMetaData, Boolean preserveVersion, Boolean discussionUpdate) at Quest.NSP.SharePoint.ClientSPListItem.SystemUpdate(Boolean incrementListItemVersion) at Quest.NSP.SharePoint.SharePointItem.Update() at Quest.NSP.SharePoint.SharePointItemWriter.WriteItem(SharePointItem item, Boolean writingPrimaryDocumentLinkFields) at Quest.NSP.SharePoint.SharePointRecordWriter.ProcessRecord(List`1 itemUrlList) at Quest.NSP.SharePoint.SharePointRecordsWriter.WriteRecords(IDataReader reader, List`1 itemUrlList, Int32 numberItems) (client workingSet = 123M)