This has been reported as bug 3308, but apparently any previously setup domain is 'grandfathered' in.
The "Username/directory/subdomain Name:" does not allow a period to be used, which means if you have a .com, a .net, and a .org, for example, you can't create the .com as user mydomain, the .net as mydomain.net and so on.
Had last set up domains with dots in the username back in late August. At that time, it successfully created the users & folders (both with dots in them).
Now, though, all domains that share the same base name - 'mydomain' - appear in the addon page to have the same user, though they still get pointed to their correct folder, & the ftp login (mydomain.org for example) still gets one into that same correct folder.
Since periods are out, one needs a different way to differentiate between similar domain names. The "Username/directory/subdomain Name:" box *does* appear to accept underscores, but the actual creation routine bombs out.
Have not tried to go through the entire character set to see what's allowed.
So - besides alphanumeric characters, does anyone know what *will* work when creating user names? Using mydomain1 for a .net is really inelegant, and counterintuitive. IMHO the username should very closely resemble the domain name.