My demo app came up with this issue after making some changes to patch config file today.
Reader is in incorrect state
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Exception: Reader is in incorrect state
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
[Exception: Reader is in incorrect state]
Sitecore.Xml.Patch.XmlReaderSource..ctor(XmlReader reader, String sourceName) +123
Sitecore.Configuration.ConfigPatcher.GetXmlElement(XmlReader reader, String sourceName) +45
Sitecore.Configuration.ConfigPatcher.ApplyPatch(TextReader patch, String sourceName) +138
Sitecore.Configuration.ConfigPatcher.ApplyPatch(String filename) +93
Sitecore.Configuration.ConfigReader.LoadAutoIncludeFiles(ConfigPatcher patcher, String folder) +177
This is a weird issue at the first glance, I still have no idea what happened under the hood, though I solved the problem.
I added a comment in my patch config which is in between and section, this is the fundamental cause.
To fix this YSOD, simply move the comment anywhere within section.
Happy coding 🙂