Ecommerce not working after moving site to new domain

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I have just moved a clients site to a new domain and the ecommerce addon has stopped working. It all looks OK but when you click buy a blank pop up appears, also all the ecommerce pages such as checkout throw a HTTP Error 500 (Internal Server Error).

The rest of the site works fine so not sure why the ecommerce addon isnt working. Anybody got any clues.

Thanks