If you are getting an Authentication Error when Logging in to the Untapped.gg Companion, your credentials might be full.
To solve this issue you can follow these steps:
1) Go to your Credential manager
2) Back up your Existing Windows Credentials
3) Try deleting several Windows Credentials (including the Untapped.gg one if exists)
4) Reboot
If after doing this your problem persists, please email us at support@untapped.gg providing the Log File. You can find the Log file here