What was reported:
Issue argoproj/argo-cd#19585 (closed, 48 comments): Since Argo CD 2.12.x, apps synced from GitLab fail with "could not read Username for 'https://gitlab.com'" from the UI. The repo credentials are configured and worked before the upgrade.
What works:
Maintainer-verified fix: resync the affected applications. Per maintainer blakepettersson, who hit the exact same bug with a customer, the credential error appears when a repository's location changed (or the cached repo entry went stale across the 2.12 upgrade) and Argo CD keeps trying the old credentials against the new URL. Open each affected app and trigger a refresh/resync so it re-reads the current repo configuration, and the username error clears. If you recently moved a repo, resync is the first thing to try before rotating credentials.