First of all, this may have been caused by multiple things, but the most common reason is you changing your Google account password which microG can't handle properly.

The most possible fix is to remove your MicroG account from your phone's "Accounts" settings, then log back in in Vanced.

https://youtu.be/S5sTXRTrD5Y

If the step above didn't fix it, FOLLOW THIS

  1. Clear DATA of both "Vanced microG" and Google Play services (Data ≠ cache). You can find these apps by heading to your device settings > applications.

  2. UNINSTALL both "Vanced microG" and Vanced.

  3. Reboot (Mandatory)

  4. Reinstall both apps [Vanced microG and Vanced) Install via vanced manager or SAI whatever you find best suitable.

    but make sure you install the Vanced microG APK before you install the YouTube Vanced app.

  5. Log in and enjoy.

  6. At last, make sure you disable battery optimization for Vanced microG using https://dontkillmyapp.com

Everything is explained in this Reddit post.