(Resolved) DOMXPath::query(): Undefined namespace prefix in /vendor/magento/framework/Config/Dom.php
I've set up a new install of 2.1.5 and migrated the default data from a Magento CE install (nothing custom was migrated). I went into my M2 settings and change the Single Store Mode value to Yes and then immediately started seeing:
DOMXPath::query(): Undefined namespace prefix in /vendor/magento/framework/Config/Dom.php on line 273. I added a try/catch in an attemtp to debug and came up with the following data shown below. However, it hasn't help me find the root of the problem.
After enabling developer tools (bin/magento deploy:mode:set developer) I was pointed to the data-migration-tool as the source of my problem. I'd edited some config files within the vendor directory when learning how to use the tool and left one of the files in a bad state. I simply removed the composer package and re-installed it replace the manipulated files.