There is an issue with an extension that is currently set up that is causing this problem.
There is an extension titled "Set stock variables" that is setting the referrer value equal to "previous_page." The problem is that "previous_page" is never defined so it contains a value of undefined. Within the EventStream template this is throwing an error because the template is expecting the referrer value to either contain a value or be empty "" (which is how our utag.js file sets this value if there is no referrer).
I would suggest removing the "previous_page" setting from the extension and add a new Set Data Values extension set up like:
Set "referrer" To: Variable -> "previous_page"
Condition if "previous_page is populated
However, I'm not sure it is recommended to alter the referrer value. I'm assuming you are trying to overwrite the referrer value for a certain vendor. I would suggest creating a new variable to capture the necessary referrer (previous_page) then mapping that new variable to the specific vendor that is needing the overwritten value.
Hope that helps.