Hi!
It is an IE border radius rendering bug, as I found out: https://connect.microsoft.com/ie/feedback/details/808920/ie11-renders-white-lines-when-border-radius-and-gradients-are-applied-to-buttons
The solution in your case is simple, luckily. Forcing RGBA coloring should get rid off that:
[html]#ajaxsearchpro4_2 .probox {
border: 1px solid rgba(255, 255, 255, 0.1) !important;
}[/html]