It's seems the bots who where focussing on Wordpress logins a while ago, are now trying the same with IPB. Lot's of faulty admin-logins from the same IP-adress since March this year.

Just wanted to give an heads up :)

There's a topic or two in the Support forums about this, although if its a consistent IP address the sensible thing would be (if its not being used by any of your membership) to consider just blocking it via htaccess.