Aaaand I got it.
I didn’t have “Json” module included in my project .Build.cs
I had just
PublicDependencyModuleNames.AddRange(new string[] { "JsonUtilities" });
When I should have had
PublicDependencyModuleNames.AddRange(new string[] { "Json", "JsonUtilities" });
Strangely, it worked just fine in 4.5.1, I wasn’t even aware of the existence of this module. I thought JsonUtilities does all the magick.
Anyways, hope this post helps someone in the future.