How to catch fancybox modal opening error?

Question

JavaScript

Good afternoon.
Implemented modal window on fancybox + ajax. My window opens normally on all devices, but users complained that the window does not open.
There is also a fact that the number of applications has dropped to almost 0 (which means the error really takes place)
I started viewing events in the Yandex webvisor and saw this there:
joxi.ru/v2947nxIkJOJmG
Please tell us what it can be and how to catch and defeat this bug in general !?
site *** - not relevant
Upper callback buttons + making an appointment
Also a huge request if you don't know how to fix the BUG, ​​but you get the same error, please inform the browser version

Answer 1

Noticed the error "Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check xhr.spec.whatwg.org/. " Try getting rid of the synchronous code in AJAX.
But forms appear.
Chrome 42, Ubuntu 14.04