Coursepress Error: Not accepting Any New Username

I'm currently making sales of a course, but alongside that I'm getting reports from a proportion of users that they can't register with the site in the first place. The signup form is giving an error stating that the username is unacceptable.

This is an intermittent error and I haven't been able to recreate it myself, but I'm receiving enough reports, and from reliable enough sources, that I do believe that it's happening regularly.

It seems to be happening only on the popup signup form which appears when clicking 'Signup' from the course detail page.

If I direct users to the normal signup page (accessed at it seems to work fine.

I have also seen reports from other WPMUDEV users that they experienced this problem.

How would we go about trying to diagnose this problem? Even if it affects only a small proportion of users, it's likely to be losing sales in the process.

Thanks for your help