opencart checkout compatibility with internet explorer

I am not really sure if this is a bug but I hope someone can look into this. When I use IE to checkout on my opencart, after every step of checkout when I clicked the continue button, it will animate and hang for a while before popping up a small window with "unknown unknown" (error msg). However, after I close the small window and click the continue button, I am able to proceed to the next step of checkout. I have to repeat this in every checkout step but I get to checkout successfully.

I tried with Firefox and Safari. The checkout process is smooth and error free! So I wonder if this is a bug on compatibility with IE? I tested it on IE9.