Just "Error" window during form filling

Hello,
Situation: I migrated the new Joomla web with Event Booking, where I use the registration form with fields, which I show depending on other list choices. I use Akeeba Backup and kickstart.php. 

Problem: When I choose the item in the list and the hidden field should appear the error window appears with the message "Error" and nothing more and the circling ring is shown on the bottom of the site.

Update: the same test here on iPhone - new last Joomla and Event Booking installation: kongres.kimit.sk/registration  .

I have no idea, what's going on, what's the problem. I found nothing in logs till now. 

Thank you for any advice, Marian

Hi Marian

I guess some kind of javascript conflict here. You can submit a support ticket sending us super admin account of your site, link to the page which we can see the issue

We are going to check to see why the issue happens and get it sorted

Regards,

Tuan