Safari wont open Instagram accounts in browser on Mac - it redirects to a blank page!

It is really frustrating as I'm developing websites and I can't test instagram links on Safari as I get a brief flash of Instagram then it redirects to a blank page with the address ending...

...instagram.com/accounts/login/?next=%2Fmy_website%2F&source=omni_redirect

Not even a login page is offered. I've cleared all caches, history and disabled content blocker and all permissions are allowed for Instagram ...but still this blank window. Everything works fine in Chrome and Firefox.

How can I fix this?