this application failed to start because side-by-side-configuration is incorrect

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