May I ask what your Debug Logs are showing?
Have you tried to log in as the integration user?
Do you have a security token?
I had the same authentication errors.
I checked the debug logs, checked the integration user credentials, regenerated a security token that was then emailed to the email account set up for the integration user. Please be mindful that if you have any other integrations using the security token, they will need to be updated with the regenerated version
I then used the integration user USERNAME, PASSWORD & SECURITY TOKEN.
I left the Debug Level as Errors Only
I left the Custom Domain blank even though we have one setup.
I ticked the Standard License Type & hit save.
https://trailhead.salesforce.com/trailblazer-community/feed/0D54S00000A8CdASAV
Please check this article.
https://trailhead.salesforce.com/trailblazer-community/feed/0D54S00000CfqokSAB