Magento – Getting Error in Magento 2.1 add to Product in Admin

This appears to be an issue with PHP 5.4+

You should NEVER rewrite core files unless absolutely necessary.

1. Create a new file:

   app/code/local/Mage/Adminhtml/Block/System/Config/Form/Field.php

2. Copy contents from

   app/code/core/Mage/Adminhtml/Block/System/Config/Form/Field.php to app/code/local/Mage/Adminhtml/Block/System/Config/Form/Field.php

3. Replace line 111 with the following: } elseif ((isset($v['value'])) && ($v['value']==$defText)) {

This should correct your error.

The ExtensionAttributesFactory (and not only) uses the dockblocks to determine "stuff". (not sure yet what exactly).
If you use OP cache, this means your php files will be cached, minified and will probably suffer other modifications.
By default the opcache does not save the comments in the minified version so $methodDocBlock = $methodReflection->getDocComment(); will return null and there is no way to find out meta data about the methods in a certain interface.

I know it's not a full answer, but the idea is in here. Magento uses the dockblocks to find out meta data about methods so you need to keep them.