I had no problem adding that product to cart, a couple of times, with or without a product with attributes already in the cart.
I do notice an oddity in view source of the product with no attributes:
HTML Code:
<!--bof Attributes Module -->
<br>
<!--eof Attributes Module -->
There should be nothing at all between the comments if there are no attributes.
HTML Code:
<!--bof Attributes Module -->
<!--eof Attributes Module -->
Perhaps you modified the code for layout purposes? The <br> is not correct XHTML ( <br /> ) and thus not from the stock code.