There is a problem with not-logged users - the products are not added to cart after clicking the button.
Please check your access rules. I tested it with the last versions and there was no problems.
Another bug (both on 5.x version) - Thickbox login does not show up. I think both issued may be related to usage of JQuery in your module and in Thickbox login.
I tested it with
* Thickbox 5.x-2.0
* Ubercart 1.3
* jQuery Update 5.x-2.0 (!)
No Problems.