Page working fine - *except* when accessed from menu...

I’ve recently updated my site, by creating a new page to replace the old one.
The page seems to work fine after extensive testing, so I “went live” with it, by publishing and updating the menu (make old-page hidden and new-page visible).
The thing is, when accessing the new page via the menu, the page doesn’t load, and the logs show an unclear error…
This is very very problematic for me.
Should I post the error? How do I go about debugging and solving this?
Thanks!!