The requested page cannot be accessed because the related configuration data for the page is invalid.

This error usually means that your applicationhost.config which is the base config installed by IIS on your windows/system32 dir which provides default configuration for .net web sites is denying override for a particular configuration section that you are using in your web.config on your web site.

This error has suddenly become very widespread because in IIS 8.0 in Windows 8.0 the default override setting for the <handlers> section is Deny which means you can't have it in your web.config. Considering that every web site template generated by Visual Studio has the <handlers> section with a few things in it, I can only fathom the logic behind this decision, other than consider it a blunder.

In any case, easy enough to fix. 

1)navigate to windows\system32\inetsrv\config and open applicationhost.config

2)find the section that you need to use in your web.config (this error usually outputs a helpful message telling which section is erroring) and change the setting from Deny to Allow.