I just got the same error in an installation. DNN 5.1.1 SQL2008 instaled languages sv dk en.
But I realized that Ithe site was running in SQL 2000 mode so I just have to blame myself 
However I changed to 2005 and did a new install and got the same error so now I dont know who to blame....
I do a new try with 2008 stay tuned..
Same result with 2008 maybe the uninstall does not work in release 5.1.1 but I dont care any more . looks like this release needs some testing....!!
SQL Execution resulted in following Exceptions: System.Data.SqlClient.SqlException: Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression. vid System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection) vid System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) vid System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) vid System.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean async) vid System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe) vid System.Data.SqlClient.SqlCommand.ExecuteNonQuery() vid DotNetNuke.Data.SqlDataProvider.ExecuteADOScript(String SQL) vid DotNetNuke.Data.SqlDataProvider.ExecuteScript(String Script, Boolean UseTransactions) DECLARE @default_constraint_name SYSNAME, @sql NVARCHAR(3750) IF EXISTS ( SELECT name FROM dbo.sysobjects WHERE parent_obj = OBJECT_ID(N'dbo.Forum_EmailQueue_TaskEmails') AND type = 'D' AND id = ( SELECT cdefault FROM syscolumns WHERE name = N'DateAdded' ) ) BEGIN SELECT @default_constraint_name = name FROM dbo.sysobjects WHERE parent_obj = OBJECT_ID(N'dbo.Forum_EmailQueue_TaskEmails') AND type = 'D' AND id = ( SELECT cdefault FROM syscolumns WHERE name = N'DateAdded' ) SET @sql = N'ALTER TABLE dbo.Forum_EmailQueue_TaskEmails DROP Constraint ' + @default_constraint_name EXEC sp_executesql @sql END System.Data.SqlClient.SqlException: Column already has a DEFAULT bound to it. Could not create constraint. See previous errors. vid System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection) vid System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) vid System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) vid System.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean async) vid System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe) vid System.Data.SqlClient.SqlCommand.ExecuteNonQuery() vid DotNetNuke.Data.SqlDataProvider.ExecuteADOScript(String SQL) vid DotNetNuke.Data.SqlDataProvider.ExecuteScript(String Script, Boolean UseTransactions) ALTER TABLE dbo.Forum_EmailQueue_TaskEmails ADD CONSTRAINT DF_Forum_EmailQueue_TaskEmails_DateAdded DEFAULT (GETDATE()) FOR DateAdded