About 1% of users getting ajax requests errors with status 0

I started to track errors from client on a specific ajax post request that send user data (basically json object and one base64 image data row) and return a processed image. If the request fails it refreshes the page.

Since it started, I'm seeing this error occures to atleast 1.5% of users, all of them are either desktop (chrome and few firefox, 8% of errors) or android (chrome, 92% of errors).

I guess there might be times that the ajax request that reports the error to the server might fail as well so it might be more than 1.5%.

The errors basically tells this story:

Ajax status code: 0
statusText: error
responseText: empty

Time since request initiated in the browser until it failed: 
between 100ms-10000ms, most of them are around 2000ms

When I looked at the country breakdowns, I discovered that out of 100% of errors, 82% of them happened in Philippines (66% of them have duplicated ip), and 13% in the US (only 2 of them were duplicated ip) this is while the overall traffic is 20% in PH and 60% in US.

So I'm thinking it might be a connectivity issue with the cellular networks but it also happens desktop which is 8% of errors, and 10% of overall traffic so i'm not sure that i just accept this answer of "connectivity issues".

I checked the web server (nginx error logs) logs and varnish logs and i found nothing that might explain this. What can I check next to make sure what the hell happened in the client? Do you have any ideas what might be the reasons to such behavior?

Thanks in advance - any help would be appreciated.