I've upgraded the SQL database of a MOSS 2007 server from 2005 express to 2008 R2 Enterprise (in two steps: SQL Express 2005 => SQL Express 2008 => SQL 2008 R2).

After the upgrade it seemed working fine, but the SQL management studio. So I uninstalled and reinstalled it, but something went wrong: the SPS cannot connect to the SQL database any more.
I see tons of errors in the event log, and I am not able to even the SPS configuration wizard.

I checked the the SQL server, and I found that there are errors on the permissons tab like this:
Couldn't obtain information about windows NT group/users 'NT Service\MSSQLSERVER', error code 0xffff0001

Please, help to find out how to repair this!

Thanks in advance!