Hello Kiba, Wilfried - Based on the snippet you have shared,
In the Step 2, it states validation is completed.
In the Step 3, it fails, and the error message states some validation has failed. When you click on the View Errors link, you are getting container does not exist. Is that understanding, correct?
Now, if there are validation error it creates a container named hnserror with an error file pointing to the potential reason of the blocker. But in case the validation succeeded this container might not have got created in the first place itself and hence the exception is then correct because container doesn't exist.
I would recommend the below:
Retry the complete Upgrade Process to see if that helps.
Review for any blocking/im-compatible/feature enabled on the account. Below is the link to refer:
If the above doesn't help, you might need to reach out to Support Team to review the backend logs and help with further troubleshooting ahead.