So we have exhausted all possible solutions with our UE4 dedicated server, multiplayer build.
We have a build with Playfab 1.37.200518 plugin with Unreal 4.24 (Oculus-branch) , VS 2017, and it works fine.
We updated the build to 4.25 (Oculus-branch), fixed the API calls error (see https://community.playfab.com/questions/41568/ue4-plugin-started-throwing-errorcode-1131.html), and used latest Playfab 1.39.200615 plugin, with VS 2019.
Server is unhealthy, too many restarts. Works well with VMAgent locally. We can see there's successful API calls, but of course doesn't go further with an unhealthy server.
We also tried to compile with VS2017, or use the Playfab 1.39.200615 plugin. Same error.
We opened a ticket, but we (and our clients) are desperate to fix the problem.
Please fix the UE4.25 plugin / integration it's of uttermost importance for us developers!