I looked at this issue with your ATS contact. We checked your environment with my Saucelabs account and that worked. Could you try to get another free trial with Saucelabs with another account and see if you get the same error message?
We can reproduce this issue if we create a Saucelabs account in which an emailadress is the username. Could you try to create another Saucelabs account, and this time not use your emailadress as username. Other thing you could try is to encode the @ symbol, which is likely causing the issue, so use %40 instead of @.
Does this solve your issue?