I've got that occasionally. Can't remember exactly but using playbook as the user name seemed to work. I also don't remember whether I left the password blank or not! The point is, you're not alone in this!
i've gotten prompted for a user & pswd anytime i check an email from my "desktop" account being that i'm BES. tried my biz login thinking it was wanting that but doesnt work. i end up either tapping an email on my other non-BES accounts or hit cancel until it goes away.
tried using admkn and playbook in multiple combo's, doesnt work for me. confirmed it only happens with my bes email account.
It happened to me when I went to buy something on a website that employed an additional authentication layer---in my case, Verified By Visa. When I submitted the order, I was suddenly greeted by a strange, primitive-looking, disembodied popup, asking simply for user name and password. Concerned about security, I kept entering nothing---or random characters---yet it kept reappearing. Finally it just went away.
I got an email from the vendor the next day, telling me my Visa purchase had been denied. It was when I contacted my Visa provider, and was told that I had failed to provide the correct Verified By Visa credentials, that I pieced it all together.
In a more general sense, the popup appears whenever "something" needs authentication.