After we migrated SharePoint from 2010 to 2013, we found several sites could not be displayed with the following error.
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Could not load file or assembly 'Microsoft.SharePoint, Version=126.96.36.199, Culture=neutral, PublicKeyToken=71e9bce111e9429c' or one of its dependencies. The system cannot find the file specified.
After looking the welcome page URL for the site users try to hit. It’s something like http://servername.domain/site/default.aspx. However, this site is Wiki sites and the welcome page should be like http://servername.domain/site/SitePages/Home.aspx.
The workaround is to activate the “Wiki Page Home Page” web feature and the welcome page will default to http://servername.domain/site/SitePages/Home.aspx.
Also this is very obvious but we had so many users complain about the site home page and this might be handy to resolve the issue for end users.