This is probably related to https://tickets.metabrainz.org/browse/BB-524 as  I recall having the same issue while creating the BB account.

So my MusicBrainz account name is spUdux, and for BB I had to change the display name to be able to create the account. Thus far I've been going by display name spUduxB but I'd really like to have it updated to spUdux. From the profile editor it does not work, hitting the Update button does nothing, there's no feedback whatsoever.