internet explorer input field not focusable

Question

I'm using the boxy modal popup in a website. I have an input field and a button right next to it. When I click the button, a boxy popup window with an iframe appears (you type in a postal code in the input and the iframe searches data for you).

When I click a button in the popup, doesn't matter what button, or an anchor tag that performs javascript in the popup, and afterwards I close the popup, the input field is not selectable anymore. It's not greyed out, so it's not disabled. It doesn't have any element positioned over it. When I close the popup without clicking a button, the input field is fine (if it was fine before opening the popup in the first place). Even weirder: when I use to cycle through all selectable items on my page, and eventually come back to my input field. It is selectable as if nothing has happened. I can select text again, type again, etc..

And guess what. Only in IE (7/8/9).

Anybody know of something..?