I am currently struggling with my Matomo instance hosted on a mutual server at OVH.
The tracker works if it is hosted on the same domain name of my instance, though it does not work if I want to track a website on a different domain.
I tried to edit the .ovhconfig but it did not help, it was set on firewall none.
I fixed my own issue, in fact OVH has another place where you can disable the firewall for the instance hosting Matomo. For this you need to access to the UI of OVH, then click on Sites and then edit the domain name in order to disable the firewall from there. So to say the .ovhconfig is useless.