Ran into the same issue, but was able to get past it by using Edge. No luck with Chrome or Safari before that.
Was able to create the sandbox but the Mail & Events pack shows up as installed but the messages failed to deliver because they are considered spam. You would think that the servers from which the Graph API calls used to seed the sample data packs were on some type of allow list... There is of course no way to re-trigger the sample data pack install