Hi there @faepifbaelf !
Thanks for reaching out here in the Shopify Community.
I’d like to clarify a couple of points here to help you to get the support you need.
The first and most important thing to clarify is that third-party gateways such as this and their integrations with Shopify are supported by the gateway providers themselves rather than by us here at Shopify.
As such, if there is in fact an issue with the integration they created for use with Shopify, it is the responsibility of the gateway provider to maintain and provide support for this.
I did have a look around online for any information relating to the error code you shared here, and did find this external guide with some info.
It looks like this error can be caused by an incorrect merchant ID, an incorrect access code, or if the order originates from an unregistered URL.
As such I would double-check the info you are using from CCAvenue (merchant ID, access code) to ensure they match your account info with them.
It’s also a good idea to clear your browser cache from time to time, and I’d also check to ensure the browser you’re using is updated and supported for use with Shopify.
Feel free to reach back out to CCAvenue so they can provide support for their gateway and integration.