I've an error while upgrading my sharepoint database with psconfig.exe

I use the following command in order to upgrade my databases:

PSConfig.exe -cmd upgrade -inplace b2b -force -cmd applicationcontent -install -cmd installfeatures

For the WSS_Content Database the command works fine. At step 2 the command crashes with the following error:

System.Data.SqlClient.SqlException (0x80131904): Login failed for User 'sp_sqladm'

The EventViewer shows the following error:

Sorry for a german error message. It means: "Error during the SQL-Database Login for 'SharePoint_Config' for the instance 10.50.10.11. Login failed for the user 'sp_sqladm'.

The user sp_sqladm has rights on the SharePoint_Config-Database:  dbowner

I also checked if the Password and the user still working. There is no problem connecting to the database using Management Studio . The SharePoint works also fine, but states an upgrade is needed.

thanks for any help. I have no more ideas at the moment.