The new release of DNN 5.1 does have a small (very small) issue when converting from V4 DNN site.  The problem exists with a key and the conversion script does not drop the key due to a naming issue.  There is a fix and is available from the support wesite. 

If you are upgrading from 4, please read.  The issue is simple to fix as even if you don't you should not encounter any problems.

 I have included Charles Nurses' comments and script below - a direct copy from the support.dotnetnuke.com site. 

There is a bug in the 5.0.0 script which renames the Primary Key from PK_EventLogMaster with no object qualifer to PK_{objectQualifier}EventLog.

Unfortunately the Schema.SqlDataProvider script introduced in 5.0 (and the basis of all 5.x new installs, 5.0.0, 5.0.1 and 5.1.0) names this key PK_{objectQualifier}EventLogMaster.

The problem is that the 5.1.0 script attempts to drop this key, and place it on the new Identity column (EventID) for performance improvements when clearing the log.

So pre 5.0 sites will get an error when upgrading from the 5.1.0 script file.

It does not affect the site - all that happens is that these sites have the primary key on the wrong column and will not get the performance gain.

This will be fixed in the next maintenance release.  In the meantime users experiencing this issue can paste this script in the Host SQL module.

/* Fix bug in changing the primary key of EventLog Table */
/*********************************************************/

--Some scenarios had the primary key as PK_{objectQualifier}EventLog
IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'PK_{objectQualifier}EventLog'))
    BEGIN
        ALTER TABLE {databaseOwner}{objectQualifier}EventLog
            DROP CONSTRAINT PK_{objectQualifier}EventLog
       
        ALTER TABLE {databaseOwner}{objectQualifier}EventLog
            ADD CONSTRAINT PK_{objectQualifier}EventLogMaster PRIMARY KEY CLUSTERED ( LogEventID )
    END   
GO