This may not be a supported type of usernames for wpmu. Did you check wpmu forums as well?
That does as an workaround, thanks 
Ok, I’ll write here a more precise description of what I did in case anyone has similar issues in the future.
There’s no restriction in registering email as an username in WPMU. The only place I’ve had trouble with the email address as the username is BuddyPress. However, there may be other places, since the @-character is tedious.
Anyway, since emails are unique in the installation, just like the usernames, they can be to uniquely identify the user logging in. The workaround the to problem is to use a separate username, and set the login procedure to use the email instead of username. Here’s a nice plugin for the task.
Of course this workaround creates a bunch of problems for me in the integration, since these usernames aren’t necessarily used elsewhere and use of the emails in this fashion is really a hack, but these issues may be easier solved in the authentication code, which is the most crucial component in the integration and where I want to concentrate all my custom settings. Or else I’ll have to figure out something else later.
For anyone concerned, I encountered the same problem reported by outolumo and can’t happen to use the workaround. I submitted to TRAC…https://trac.buddypress.org/ticket/1356. Will see what happens!
The resolution from apeatling (in response to https://trac.buddypress.org/ticket/1356):
If you are allowing @ symbols in usernames, then please add this to your wp-config.php:
define( ‘BP_ENABLE_USERNAME_COMPATIBILITY_MODE’, true );
https://codex.buddypress.org/how-to-guides/changing-internal-configuration-settings/
Worked for me!
Note that this ticket has been reopened because there are additional problems with this solution, like doing bp actions in which a @ sign user is the subject (edit profile, possibly others).
I’ve tracked this problem down I think: wp_sanitize_redirect is stripping out @ signs …
Override this function ( write it in bp-custom or your plugin ) if you want to use emails as usernames. Be careful, you don’t want to just comment everything out because there are security issues to worry about.
I also use the email login plugin, but generate a username from the fullname using Javascript (link is for blogname from blog title, but the principle is the same). I think that’s a cleaner solution that avoids a lot of the issues you ran into.
