Hello. I updated to Forums 04.05.03 and I'm getting this error when I click on "My Settings" and "PM" in the forum menu:

A critical error has occurred.
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

SqlServer 2005 express.

This is the error log:

AssemblyVersion: 04.08.04
PortalID: 12
PortalName: *********
UserID: 38
UserName: FerNy
ActiveTabID: 315
ActiveTabName: FORO
RawURL: /FORO/tabid/315/ctl/UCP/mid/704/view/7/Default.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://*************/FORO/tabid/315/Default.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; InfoPath.1; .NET CLR 2.0.50727; OfficeLiveConnector.1.3; OfficeLivePatch.0.0)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: f5a3385d-44be-4dfd-a217-4dc0195d344e
InnerException: La subconsulta ha devuelto más de un valor, lo que no es correcto cuando va a continuación de =, !=, <, <=, >, >= o cuando se utiliza como expresión.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Data.SqlClient.SqlConnection.OnError
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: La subconsulta ha devuelto más de un valor, lo que no es correcto cuando va a continuación de =, !=, <, <=, >, >= o cuando se utiliza como expresión. ---> System.Data.SqlClient.SqlException: La subconsulta ha devuelto más de un valor, lo que no es correcto cuando va a continuación de =, !=, <, <=, >, >= o cuando se utiliza como expresión.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.SqlDataReader.ConsumeMetaData()at System.Data.SqlClient.SqlDataReader.get_MetaData()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.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior)at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(SqlConnection connection, SqlTransaction transaction, CommandType commandType, String commandText, SqlParameter[] commandParameters, SqlConnectionOwnership connectionOwnership)at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, CommandType commandType, String commandText, SqlParameter[] commandParameters)at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, String spName, Object[] parameterValues)at DotNetNuke.Modules.Forum.SqlDataProvider.PMGetMessageStatus(Int32 UserId, Int32 PortalId)at DotNetNuke.Modules.Forum.PMController.PMGetMessageStatus(Int32 UserId, Int32 PortalId)at DotNetNuke.Modules.Forum.UCP.Menu.CreateChildControls()at System.Web.UI.Control.EnsureChildControls()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Control.PreRenderRecursiveInternal()at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)--- End of inner exception stack trace ---
Source:
Nombre del Servidor: ***********

I couldn't reproduce your issue on DNN 4.9.4.  I installed there a new Forum 4.4.3 and doing some test posts, then I upgraded to Forum 4.5.3 and all worked well.  I have there installed also 2 other languages. 

I think it happend with any issue on previous forum upgrade to 4.4.3 with the iUpgradeable feature.  I want to like to say check all the forum 4.4.3 database artifacts before upgrade to 4.5.3.  If it is possible make a DN 4.9.4 install on a local maschine with the forum 4..3 module, and check against the datbase artifacts of your live system using the SQL Managaement Studio (Express).

If you find any differences solve it and it would be great to report here.

Finally I found a solution: You have to clean the DB tables related to the PM (3 tables). You can do it with SQL  from Host Menu/SQL.