I have removed all cookies, etc. as suggested and am still unable to access premium upgrade. Please advise.
please clear your browser's cache and application data. In Chrome, in Advanced Settings -> Content Settings -> All cookies and site data, find todoist.com on this list and delete it. In Firefox, go to Options -> Advanced -> Network tab where Todoist will be listed at the bottom.
If this won't help, please try to use Chrome's Incognito window (Ctrl+Shift+N) or Firefox's Private Browsing mode (Ctrl+Shift+P) and see if you can access Premium features through there.