Attempts to create users in the adobe admin portal causes the below issue. It's been this way for a few days and I've tried Safari and Chrome.
Further details: This appears to happen when creating a Federated ID user. Standard "Adobe ID" seems to generate without issue.
If you are using Azure sync then the users will be managed from the IDP console. You can add the users in the Azure console which will add those accounts to the Adobe admin console.