For a successful connection to Snowflake, the URLs listed in the Snowflake allowlist should be accessible via the browser. To verify, you should try to access the URLs in your browser. The results of this browser test can help Snowflake understand the possible root causes of your issue.
For the Snowflake URL, you should be able to access the login page in the browser.
For a Stage URL, you should expect some sort of 403 error, but still succeed in connecting. See the examples below:
<Error><Code>InvalidQueryParameterValue</Code><Message>
Value for one of the query parameters specified in the request URI is invalid. RequestId:1c0658d7-e01e-010c-5be8-8023af000000 Time:2022-06-15T18:44:55.1523344Z
</Message><QueryParameterName>comp</QueryParameterName><QueryParameterValue/><Reason/></Error>
GCP stage successful connection example:
<Error><Code>AccessDenied</Code><Message>Access denied.</Message><Details>
Anonymous caller does not have storage.objects.list access to the Google Cloud Storage bucket.
</Details></Error>