Günther, smells like you're using very strict privacy settings like Private Browsing, "do not track", no cookies allowed, flush the cash when tab or session is closed, etc, etc.
For NG to work properly the nikongear.net cookie needs to be installed but also accessible by the browser, when for instance using Safari in Private Browsing mode new cookies are not installed but already installed cookies are also ignored. The newer web browsers are now designed to have each tab to be a separate browser session, with a super secret privacy setup this might cause login failures.
As we're constantly updating the site a periodic cleaning and re-installing of the cookie is advised to guarantee a smooth experience, but this goes for all websites.