Had same issue and could get to AppStore Connect through https://developer.apple.com as well.
I think issue might be launching the old https://itunesconnect.apple.com/ URL in the browser leaves you stranded on the "Get Started with iTunes Connect" page.
Try using the new https://appstoreconnect.apple.com instead.
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect
Tags: