It is caused by a Plugin you have installed. Disable all plugins and enable them one by one (check each time to see if you get the error) to find out which one is causing error.
You have a plugin located in your Plugins & Products > Plugins which is using the fetch_userinfo hook. This error is caused by a Modification you added. So, as suggested, go disable your products one at a time and see if this corrects things. You will then figure out which product/modification is causing the problem.