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

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.