I encypted the appconfig like this , on my computer works well but on a network doesn't work
Configuration config = ConfigurationManager.OpenExeConfiguration(exeConfigName);
            ConnectionStringsSection section =
            config.GetSection("connectionStrings")
            as ConnectionStringsSection;
            if (!section.SectionInformation.IsProtected)
            {
                section.SectionInformation.ProtectSection(
                    "DataProtectionConfigurationProvider");
                config.Save();
                ConfigurationManager.RefreshSection("connectionStrings");
            }