Couldn't sign you in This browser or app may not be secure. Learn more Try using a different browser. If you’re already using a supported browser, you can refresh your screen and try again to sign in. STEPS TO REPRODUCE Tried to sign into YouTube OBSERVED RESULT Couldn't sign you in This browser or app may not be secure. Learn more Try using a different browser. If you’re already using a supported browser, you can refresh your screen and try again to sign in. EXPECTED RESULT SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: (available in About System) KDE Plasma Version: KDE Frameworks Version: Qt Version: ADDITIONAL INFORMATION
Try to change the user agent to some new Firefox or Chrome. This thing is forced by the service you mentioned, please fire a bugreport there. *** This bug has been marked as a duplicate of bug 427133 ***