Signing into the Nexus 3 UI with default admin user may not make the admin cog visible. The UI believes you have no session even though the signin was successful.
The problem happens using regular browser windows. It may be related to signing into different versions of Nexus repository Manager and getting HTTP sessions for them at the same host URL. For example perhaps some resource had been cached in browser from older version that prevents proper resource fetching of newer 3.12.0 version.
With incognito window, all subsequent requests sent from the Nexus UI send the NXSESSIONID cookie you receive from the signin - and therefore the admin cog displays and you have a valid session.
NOTE: previous workarounds of signing in then signing out ( to delete NXSESSIONID cookies ) do not work.
Unsure if this is a Chrome browser bug or UI timing issue finding the cookie received from the signin or something else.
Follow both these steps:
1. Clear browser cache for the domain that you are accessing Nexus on, possibly using a Chrome extension like this one: https://chrome.google.com/webstore/detail/clear-session/maejjihldgmkjlfmgpgoebepjchengka
2. Restart the browser ( only doing step 1 will not work ).