Hi there, I have read the description and the error seems to be on your tracking code from google, it maybe having a conflict with your add to cart or it is adding parameters to your URL that are giving you a backend error, I might be abble to debug. on the Propostal I have set 10 days but I can do it in less depending on the opencart version and if it has or not the VC extension, also I will need access to your host pannel to make backups, since I'll be changing code it is good to always keep backups, also I might need to know information about the shopping time to know the time when less visitors come to your website, to be sure that I won't create any bad experience for the users at the time of a purchase.