Help: Error Code 1147405308 (Authentication Failed) – Nothing works!

Since today I've been getting error 1147405308 ("Access Denied") when trying to log in. My internet is working perfectly fine otherwise.

Here is what I have already tried with no success:

• Restarted PC & Router (Hard Reset)
• Verified integrity of game files
• Changed DNS manually & disabled IPv6
• Checked Windows Firewall
• Tested connection via mobile hotspot (same error!)
• Completely reset network stack (Winsock/IP) via CMD
• Deleted the Anti-Cheat folder and let Steam redownload it

Does anyone have any other ideas? Appreciate any help!