That error seems to occur because one of the parameters set by the EU VAT Assistant is missing from the page. The plugin sets a list of parameters, which are stored in a variable called aelia_eu_vat_assistant_params. One of them is called aelia_eu_vat_assistant_params.user_interface.self_certification_field_title, which is assigned to a variable called self_cert_label. That’s the variable that the script is trying to process and, for some reason, such variable is not set.
This might be happening because something alters or removs the EU VAT Assistant parameters. It could be another plugin, or a JavaScript minification service (JavaScript minification can have side effects).
Finding the exact cause would require an investigation on the affected site (this would be possible under one of our support plans). Alternatively, disabling the minification of scripts might help. You could also try to disable other plugins, to isolate the source of the conflict.
