Having the same issue as Chris but my custom settings is with the correct email address and location profile = system admin. The einstein vision  package was successfully installed and I also validated the previous challenge about setting up the einstein account so really not sure what s missing here. Any idea ?

hi all, we are enabling lightning experience in our sandbox. We have gone through the creation of "my Domain" and its deployment (which has been successful a week ago so no domain propagation issue here). Now in SF Classic, the URL contains our custom domain which is great.

However, as soon as a user tries to switch to lightning experience, we have a "server not found" message. From then, they basically cant access in URL in the sandbox and the only way to "unlock" them is to remove then from the selected users list (avialable from LEX menu in SETUP).  Have logged cases with SF without success so hoping the broader community can help :). Did anyone experienced a similar behaviour ? Are we missing a basic step?
Please note the custom domain is not created in the production yet

thanks all for your help