If your account was recently created and you are using the Ultra plan, you may be randomly shadow banned (someone it might not happen).
For shadow banned accounts, the frequency of the “high traffic” error seems to vary depending on the duration (probably).
An account created yesterday will experience the error 9 to 10 times out of 10.
An account created 30 days ago will experience the error 2 to 3 times out of 5.
Accounts that are not shadow banned never fail, not even once out of 100 attempts, regardless of the model used (as long as the quota permits).
Shadow banned accounts will encounter a 403 error when using the Gemini CLI (which is different from a ToS violation).
Based on the information gathered here, only a very small number of internal engineering teams are aware of this shadow ban.
The Google One team and the Google Cloud team—especially their customer support organizations—are completely unaware of it.
That is why they do not provide an answer.
In fact, they cannot answer.
They do not even know the cause.
It seems the shadow ban might be lifted 60 days after it is applied, but this is not certain.