In Appearance > Elements – do you have elements?
If so Quick Edit all of them and change the status to Draft. This will disable them.
If this resolves the issue then we know it is one of them elements – which you can find by enabling one at a time.
“Merge Header” is conflicting. And it looks like the only issue that occurs when disable this element. Is that the nav-menu sub menus aren’t displaying when hover above it.