Lot of events in the log .... below is the event log.

System.Data.SqlClient.SqlException: The DELETE statement conflicted with the REFERENCE constraint "FK_yaf_UserPMessage_yaf_PMessage". The conflict occurred in database "discussionforum", table "dbo.yaf_UserPMessage", column 'PMessageID'.
The statement has been terminated.
at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection)
at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)
at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)
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 YAF.Classes.Data.DBAccess.ExecuteNonQuery(SqlCommand cmd, Boolean transaction)
at YAF.Classes.Data.DB.pmessage_delete(Object userPMessageID, Boolean fromOutbox)
at YAF.Classes.Data.DB.pmessage_delete(Object userPMessageID)
at YAF.Controls.PMList.DeleteSelected_Click(Object source, EventArgs e)
at YAF.Controls.ThemeButton.OnClick(EventArgs e)
at YAF.Controls.ThemeButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)