PHP Storm just notified me that in the class-metabox.php file of the GPPremium (V: 1.11.3 ) plugin at line 1259 there is a </body> instead of a </tbody> inside the table.generate-elements-settings.
I just re-downloaded v1.11.3 and it is definitely still there at the same line 1259. Probably it was fixed in 1.11.0 but it might have been reverted by mistake π