When trying to delete Documents i'm getting the following error:

System.Threading.ThreadAbortException: Thread was being aborted.
   at SNIReadSync(SNI_Conn* , SNI_Packet** , Int32 )
   at SNINativeMethodWrapper.SNIReadSync(SafeHandle pConn, IntPtr& packet, Int32 timeout)
   at System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()
   at System.Data.SqlClient.TdsParserStateObject.ReadBuffer()
   at System.Data.SqlClient.TdsParserStateObject.ReadByte()
   at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
   at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe)
   at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()
   at Umbraco.Core.Persistence.Database.Execute(String sql, Object[] args)
   at Umbraco.Core.Persistence.Repositories.PetaPocoRepositoryBase`2.PersistDeletedItem(TEntity entity)
   at Umbraco.Core.Persistence.Repositories.ContentRepository.PersistDeletedItem(IContent entity)
   at Umbraco.Core.Persistence.Repositories.RepositoryBase`2.PersistDeletedItem(IEntity entity)
   at Umbraco.Core.Persistence.UnitOfWork.PetaPocoUnitOfWork.Commit()
   at Umbraco.Core.Services.ContentService.Delete(IContent content, Int32 userId)
   at umbraco.cms.businesslogic.web.Document.DeletePermanently()
   at umbraco.cms.businesslogic.web.Document.delete(Boolean deletePermanently)

I searched the web to delete Documents properly and this is what I use to delete documents:

// first check if published, then unpublish
if (d.Published) {
    d.UnPublish();
    umbraco.library.UnPublishSingleNode(d.Id);
}

// delete permanently
d.delete(true);

According to the logs when the error occurs:
The document is published and gets unpublished
And when trying to delete it i'm getting the error above.

I have no idea why the error occurs and hope to get it resolved by posting it here.

Can somebody tell me if i'm doing something wrong to delete a document or is there another way to delete documents using Umbraco 6?

PS: I'm using Umbraco 6.0.3