Ruled out (all tested, none fixed it):
4 different browsers (Chrome, Edge, Brave, Firefox) + Incognito
Fresh Windows reinstall (2 days old)
DNS flush, Winsock reset, VPN
Hosts file — clean
Third-party cookies allowed, explicit exceptions for google.com, youtube.com, accounts.youtube.com
Full cookie wipe + direct sign-in attempt
Confirmed cookies ARE saved after sign-in (not being blocked)
Different device entirely (mobile Chrome, forced desktop site) — same failure
Security Checkup: all green
Recent activity: all sign-ins show "Recognized," nothing flagged
Approved a "new device" prompt — no change
Fails identically on two separate devices, so it's not local hardware/network/browser. Looks like an account-level restriction on YouTube sign-in specifically. Please escalate to someone who can check for an account-side hold or restriction on YouTube sign-in access. Google ai failed me. I used ai to help diagnose and write this for transparency sake.
Source: r/GoogleSupport · by /u/Many_Independence685