Mobile Menu doesnt work after updating wordpress

Hello!

I took over a webpage for maintenance shortly before somebody updated wordpress to 5.3.1.

Everything is working just fine, except the mobile menu.

(Theme architecture)

Its possible to select entries from the mobile menu, but then nothing happens, no linking to the next page.

What do you need from me, f.e. code?

Thanks in advance for your fast help!

Sorry I misspressed a button -> there is no solution!

Why is nobody answering after weeks-->where is the support, paying customer...