Everything on my end was working fine and suddenly I faced this issue while hitting my SharePoint Application. This was the below error which I was getting while hitting the app.
After going through some of the forums I got to know that one extra line has been added somehow in <appSettings> for key. When I removed this line, I was able to hit my application without any issue.
Incorrect:
<appSettings configSource="AppSettings.config">
<add key="aspnet:UseStrictParserRegex" value="true" />
</appSettings>
Correct
<appSettings configSource="AppSettings.config">
</appSettings>
