So I installed a test environment and deactivated GP Premium and lo and behold: the error does not appear in the console anymore. I have been looking for this error for 3 hours now. Is it possible that it is still cached somehow?
Hooks is now a deprecated module – it was replaced by Elements.
Since you have Elements activated as well, you’ll want to go to “Appearance > Elements” and click the “Legacy Hooks” button at the top of the page. That’s where you’ll want to remove your old hook.
Alternatively, you can just deactivate the old Hooks module.