We have a 4011 running on 6.44 firmware. “admin” username’s password is blank. today i was tried to login via winbox, but winbox returned login and password error. 4011 is running on my local network, no one can access them… i try to login via ssh username=admin password=admin and it works, but when i try to login via winbox with user/pass=admin, winbox again returns me login/pass error. Is it firmware bug?
sorry for bad English 
Try another browser: I had similar problems on my CRS317 and CRS328-24P switches: firefox could not connect, but Chrome did work. Today it works again, no idea why
I think that MT must have some built in smart software that after one logs in so many times without assigning a password, it figures the admin isn’t qualified enough to use/config the router. 
Resseting configuration should not be allowed without setting password as integral part of this process.