On the careers.php you have the bootstrap include files... they also exist on the template, so they were being added twice and breaking the page.
I removed the reference on the careers.php page and the about link functions again.
The link path is wrong in the Templates/Primrose_Template.dwt.php file. On line 287 you have the href="About_us.php". Since your page is in the Templates folder it has to be: href="../About_us.php"