it could be other things but in my case (which is common apparently) is was just caused by having an ampersand (&) within one of the values in the web.config/app.config appsettings key-value pairs

the fix is to simply change & to &

this is because the xml reader is not able to parse the *.config file properly unless the value is xml encoded. this applies for these characters as well (and their correct/encoded representation)

"   "
'   '
<   &lt;
>   &gt;
&   &amp;

there could be various other complex reasons (you can search for the full string online and you will find them)

but hope that helps

The post this Application Failed to start because side-by-side-configuration is incorrect appeared first on Ryan Garaygay.