In all the cases that I have seen this, it's due to the `App_Config` directory containing some kind of problematic config chunk like this:

`<index id="xxxxxxx">...</index>`

The issue may not be with the config itself, but with the current state of your Solr indexes.

Using your favourite text editor, search all Sitecore config files in `App_Config` for `<index `

Start by commenting out any custom entries, one by one, and seeing if the site will load. This should help you locate which index is causing the problem. While you are doing this, be sure to verify that the index folders that you are expecting in `C:\Solr\solr-x.x.x\server\solr` exist, are well formed, properly named, and not missing any files; particularly config files in these directories `C:\Solr\solr-x.x.x\server\solr\MyIndex_core_index\conf`. 

Also check that the `core.properties` file `name` attribute matches the core folder name.

If you're seeing this error when doing a SIF install, it's probably best to uninstall completely and then start over. Before trying again, check all of your `.json` SIF config files for references to what looks like custom indexes. Remove them and keep trying.

I have also seen this issue after building a cloned solution to the web directory. The issue was specifically with a Coveo index config, whereby a `.example` config file needed to be enabled.