Subpages under start page not working & returns json {"result":false}

We are in a process of upgrading our CMS & Commerce to latest version CMS 12 & Commerce 14.

We are facing a strange issue. 

Pages under startpage on browsing not working & returning json {"result":false}

For Example : I have startpage with url name as “start”. Under StartPage there is standardpage “About-us”.

When browsing www.sitename.com/about-us/ returns json {"result":false} but when we check inside cms edit it works fine.Or if we append startpage url for example  www.sitename.com/start/about-us/ it works.

Also I tried adding simple address to “About-Us” page but even simple address is not working and even simple address returns json {"result":false}