I have a SharePoint 2013 installation, current through the April 14, 2015 cumulative update for SharePoint Foundation 2013 (KB2965261) patch. In Central Admin, I am trying to toggle the authentication provider for the "Sharepoint - 80" web app from Forms Auth to Windows Auth (sidenote: this was my original intent; the error actually occurs if I just open the Authentication Providers dialog, edit the Default, and hit Save without changing anything).

With the web app selected, I go to Authentication Providers, select Default, then hit Save on the ensuing dialog box. It sits there for a couple minutes, then gives an error:'<', hexadecimal value 0x3C, is an invalid attribute character. Line 6, position 28. I found the error in the log, which led to this stack trace:

Application error when access /_admin/Authentication.aspx, Error='<', hexadecimal value 0x3C, is an invalid attribute character. Line 6, position 28.   
at System.Xml.XmlTextReaderImpl.Throw(String res, String[] args)     
at System.Xml.XmlTextReaderImpl.ParseAttributeValueSlow(Int32 curPos, Char quoteChar, NodeData attr)     
at System.Xml.XmlTextReaderImpl.ParseAttributes()     at System.Xml.XmlTextReaderImpl.ParseElement()     
at System.Xml.XmlTextReaderImpl.ParseElementContent()     
at System.Xml.XmlLoader.LoadNode(Boolean skipOverWhitespace)     at System.Xml.XmlLoader.LoadDocSequence(XmlDocument parentDoc)     
at System.Xml.XmlDocument.Load(XmlReader reader)     at System.Xml.XmlDocument.Load(String filename)     
at Microsoft.SharePoint.Administration.SPAspConfigurationFile.MergeWebConfig(XmlDocument xdWebConfig, String fileMask)     
at Microsoft.SharePoint.Administration.SPAspConfigurationFile.Copy(FileInfo srcFile, DirectoryInfo destDir, String validationKey, String decryptionKey, Boolean bMergeWebConfig, Boolean bBackupExistingFile)     
at Microsoft.SharePoint.Administration.SPWebApplication.InstallFiles(DirectoryInfo vrPath, String validationKey, String decryptionKey)     at Microsoft.SharePoint.Administration.SPWebApplication.Provision()     
at Microsoft.SharePoint.Administration.SPWebApplication.UpdateAuthenticationSettings(SPUrlZone zone, SPIisSettings newSettings, Boolean createJob)     
at Microsoft.SharePoint.ApplicationPages.AuthenticationPage.BtnSave_Click(Object sender, EventArgs e)     
at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)     
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I'm looking for how to solve this, or at least find what is causing the problem. I think first, is there any documentation on what SPAspConfigurationFile.MergeWebConfig does? I'm assuming that it's doing something with web.config files in the web app, but what? And which ones? Or is there documentation about what is going on behind the scenes in this process that would help debugging? I do have the full log file if there is something else that might be of use.

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