Error: Cannot create a user with an empty login name.

Thanks for reaching out @arosier, and for sharing details on the steps you performed to troubleshoot this. Please share the following and we can investigate this further:

1. Your Site Health information. You can use this form to share privately if preferred.  
2. A screen recording of your experience. You can use a service such as Loom or RecordIt to share a screen recording, while using the same form to share this privately by adding a link to the recording in here. 

When making a recording, please do so from a Chrome browser incognito window. Depending on what we can see, we’ll provide you with some further troubleshooting suggestions.

Let me know if you have any questions with the above.