The problem is that the behaviour.js when trying to upload using the ecommerce module in 2.3.1 returns this error in IE8.
I am aware that the client using SS 2.3.1 is a IE5/6 user thus different problems have to be fixed in the css but this truely is a .js error.
f.y.i. the file behavior.js is in the jsparty folder...
I was able to resolve this issue by using the compatability view under the tools menu in IE8. This is however something the SilverStripe developers should look into as I do not believe this is a good long-term solution.
My JS knowledge is crappy so I can't help you out but I thought I'd just let you know that Behaviour.js is in the process of being totally rewritten and they're doing away with the jsparty folder for the 2.4 release so I imagine there isn't going to be any time to put into fixing this bug if it can work under compatibility mode as it's going to be a different process for 2.4.
Here's hoping that it's not to long until next release :)