I received error from a user when they try to install c# application, and it throws error text like

Failed to save settings: The configuration file has been changed by another program. (C://Users//USERACCOUNT//AppData//Local//APPLICATIONPATH//user.config

Stack trace:
`An unhandled System.Configuration.ConfigurationErrorsException exception occurred at:
at System.Configuration.MgmtConfigurationRecord.SaveAs(String filename, ConfigurationSaveMode saveMode, Boolean forceUpdateAll)
at System.Configuration.ClientSettingsStore.WriteSettings(String sectionName, Boolean isRoaming, IDictionary newSettings)`

This caused a downstream System.Configuration.ConfigurationErrorsException exception at:
at System.Configuration.ClientSettingsStore.WriteSettings(String sectionName, Boolean     isRoaming, IDictionary newSettings)  
at System.Configuration.LocalFileSettingsProvider.SetPropertyValues(SettingsContext context, SettingsPropertyValueCollection values)
at System.Configuration.SettingsBase.SaveCore()  
at System.Configuration.SettingsBase.Save()
at System.Configuration.ApplicationSettingsBase.Save()
at Advisor.Ui.App.App_Startup(Object sender, StartupEventArgs e) in App.xaml.cs:line 150
at System.Windows.Application.OnStartup(StartupEventArgs e)
at System.Windows.Application./.ctor/b__1_0(Object unused)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)

at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

From stack trace, I can say this happened when tried to change user.config file.
But unless the application is installed, the user cannot see this file. Then how it could be modified?

This error throwing for very few and not a common issue. How this error thrown in specific cases?
How to troubleshoot this?