Wait, what?! You are faster than a flash of lightning as the germans say! Thank you very much for this superfast answer.
Yes, it is the plugin “Meta Tag Manager” 2.0.2 from Marcus Sykes that interferes with yours! After deactivating it, the quotation marks are displayed as configured in your plugin. Since I dont need it anymore, I’ll simply delete it.
I wish you all the best for 2017, pepe!
wpressman