Mantis#1726. Thank you kindly, StrawberryFride for a patch that:

On an MSSQL-based Grid OpenSim installation, users could log in to the sim once, 
then log off - after a short time before retrying users would be unable to log in, 
and would see an empty alert box on the client with just a "close" button and no text. 
Despite no users being logged into the sim, user server would report a higher number 
of logins than logouts.

OpenSim/Data/MSSQL/MSSQLGridData.cs

@@ -149,6 +149,10 @@ namespace OpenSim.Data.MSSQL
             IDataReader reader = null;
             try
             {
+                if (database.getConnection().State == ConnectionState.Closed)
+                {
+                    database.Reconnect();
+                }
                 Dictionary<string, string> param = new Dictionary<string, string>();
                 param["handle"] = handle.ToString();
                 IDbCommand result = database.Query("SELECT * FROM " + m_regionsTableName + " WHERE regionHandle = @handle", param);