I've been building this site, few months ago I took a break and now I'm at it again and I have already faced an issue which I have no idea how to solve. When I re-started working again, I dropped the DB and newly installed the CMS.

CMS doesn't install a template if it's named index.php, but loads the contents without errors. Everything works if I rename it to something else, like index_.php. All other templates are loaded just fine though...
It doesn't even load if it's just an empty template either...

Am using CouchCMS 2.1 with PHP 5.6.

Right. I appears that after such a long time I completely forgot about the addon "simple-access-control" which let's you show templates in the admin panel menu for specific users. It was configured for a different username than the one I was always creating...