SharePoint 2007 - 2010 in place upgrade - Fail to migrate databases from previous major release's standalone configuration

Hello, 

I'm trying to do an in place upgrade from SP 2007 to SP 2010 and after running the SP2010 setup, the configuration wizard starts and on the second step an exception is thrown.

Exception: Microsoft.SharePoint.Upgrade.SPUpgradeException: Fail to migrate databases from previous major release's standalone configuration.
   at Microsoft.SharePoint.Upgrade.SPManager.BootStrap(Guid sessionId, SPUpgradeOperationFlags flags)
   at Microsoft.SharePoint.PostSetupConfiguration.UpgradeBootstrapTask.Run()
   at Microsoft.SharePoint.PostSetupConfiguration.TaskThread.ExecuteTask()

I've tried everithing I found over the web. Also, this is my second attempt, and before the upgrade I run all the service packs and the tools to set up the servers.

Anyone have any idea of the source of this error?

Thanks in advance!