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.
object(Magento\Framework\Config\Dom)#313 (7) {
["validationState":"Magento\Framework\Config\Dom":private]=>
object(Magento\Framework\App\Arguments\ValidationState)#318 (1) {
["_appMode":protected]=>
string(7) "default"
}
["dom":protected]=>
object(DOMDocument)#316 (34) {
//Edite: unuseful data after realizing solution
}
Edit:
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.
