Email tracking does not work

Tried to find my case in this forum but no luck. Could you advise, please?

Your software
My Mautic version is: 2.16.3
My PHP version is: 7.0.33

Your problem
My problem is: when email is sent to contact it contains a tracking 1x1 gif with unique url like https://appem.cc/index.php/email/5f15cbd96c4f2402160049.gif

The browser loads the pixel, the Mautic server returns 200 OK, but no data appears on the contact page.

These errors are showing in the log: none

Steps I have tried to fix the problem: made sure Mautic nginx indeed gets the requests and sends 200 OK response.

Thank you.