Hi @lynxm,
thank you for the links. I’ve checked both sites, and it appears that the on the page where the menu is not working, there are two DOMContentLoaded. The second one seems to be fired by FlyingPress (I think so), because in the code, there is a FlyingPress class.
The event that fires from that script:
Can you try that by disabling FlyingPress, and if that thar does not work, can you disable all other plugins as well, starting with optimization & caching ones.
Let me know please.
Thanks,
Matej