Cant add menu items with my theme

I use the understrap theme as a copy from the the original site, where the images and all other linked things were on the old host. Then I changed the theme and all was working and being showed like I wanted.

Then I tried to put that theme on the original site, but when I do it, all is working fine, except adding menu items or doing other things with the menu. Then the admin ajax will throw a 500. If I change theme it’s working.

And now to my question: How can I find the part of my theme that is messing with WordPress?

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