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");
}