This sounds exactly like the issue I've been having with my company's instance of Nexus Repository 3 for the past several months. It's a SPA, so I can "log in"

This sounds exactly like the issue I've been having with my company's instance of Nexus Repository 3 for the past several months. It's a SPA, so I can "log in" and the UI reflects being logged in, but all of the API calls to the backend return a 403. However, this doesn't happen if I use private mode.