Thought I would jump in here as I have never tried to add a menu item that way … didn’t even know one could do so 🙂 Cool!
That said, I have just now tried doing so with the latest of everything, WP, GP, GP Premium (everything enabled) and running on both a Linux shared host as well as locally with XAMPP. No problems on either. Heck, I could even add a Lifter LMS Course on the live site 🙂
Would be interesting to find out what’s going on. One thing though, I noticed you had WP Super Cache; did you flush it before deactivating it? I don’t have any experience with that one, but on one site I was doing (trying to) some updated on, WP Rocket was making me pull out what little hair I do have left! I used every “clear cache” link I could find and then deleted it; then things worked 🙂
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