I've seen this problem too.
Happens when 'R' users try to unleash lots of concurrent logins.
I just checked - this property must be on by default:
1> sp_passwordpolicy 'list', 'enable last login updates'
2> go
Policy_option Curr_value Min_value Max_value
------------------------------------ ---------------- --------------- ----------------
enable last login updates 1 0 1
I'm turning it off to see what happens, or doesn't as the case might be.