The Visa Checkout Sandbox Environment is properly functioning. I recommend you to attempt login at: https://developer.visa.com/
If you are experiencing an issue, please share a bit more detail as to what is happening. Also, to further investigate, please provide the following information:
1. End Point
2. Request Header
3. Request Body
4. Response Header (include the x-correlation-id)
5. Response Body
Using SoapUI, you can find the x-correlation-id in the Raw Tab of the response header.