Search doesn’t return anything

search bar on skorpion.info.pl doesn’t return anything – it sends you to a page "skorpion.info.pl/undefined". I tried working on that, but couldn’t fix it in any way. The search is run by Advanced Custom Fields and ACF: Better Search. It somehow worked ONE TIME, but doesn’t anymore.

Can you please take a look? What is wrong with it? What could make it work?

Best regards