@librious, @tigerman325 and everyone else plagued by the “not logged in on release import” error, web browsers are changing when cookies are being sent.
If you import a release with one of the many userscripts or a-tisket MusicBrainz depends on a specific cookie in order to identify the user otherwise you can’t add new data (except CD stubs).
As a Chrome user you can revert to the old behavior by setting browser policies.
[*.]musicbrainz.org to LegacySameSiteCookieBehaviorEnabledForDomainList
Users of a Windows Home edition can use Policy Plus as an alternative to the group policy editor.
Restarting Chrome should make the policy take effect. This will also be indicated in the Chrome menu.
The reason why logging in again works is because the cookie date is being refreshed and thus triggers the “Lax + POST” mitigation.
Firefox has not yet enabled the new behavior by default (except for Nightly) but it will happen sooner or later.