Server is in script upgrade mode
If one day you see this message above when you try to login in SQL Server, you don’t need to care about it. This message can appear i think just in a very specific moment when you update the SQL Server service pack, yep, that is you are reading. After you update the SQL Server pack, i.e. SP1-> SP2. After you update, if you try to access the SQL Server instance it won’t be possible for a few seconds, because in background SQL Server are creating new system stored procedures, system management views and so on, and you can confirm that just going into SQL Server errorlog and seeing what SQL Server is doing at this moment.