When it go something go wrong via AJAX where find info

[PepaPolach]

Hello,

when I want to update something and I have debug mode on, it shows me symphony exception so I can manually fix the problem.

But when I want to add something what is under AJAX saving e.g. in products section sometimes is throws a red error in right top corner and THERE IS NO CHANCE TO FIND OUT WHERE IS THE PROBLEM. visible in attached image

Is there a place in file system or in DB or anywhere else where I can find the issue? 

Right now I am trying to generate combination of one of my products, but I am not able to do it unless I fix something, but I do not how to proceed. 

Can you help please? 

[Rhobur]

did you look into the browser's dev tools, the Network tab, XHR selected?

It shows you exactly this.