Hey, thanks for still sticking around, trying to help!
I tried building in the command line shell. Unfortunately, and funny enough, error messages and no explanations still popping up.
What is funny, is that when I check my installled boards (pio boards --installed) I get to see my custom board in all of the platforms: in nordicnrf52, espressif8266, atmelavr and native. All these platforms for some reason list my custom board in there.
EDIT: Hey, I think I got it! Well, it seems that platformio prefers to have its variants in the original folder, that is C:\Users<user>.platformio\packages/… Only when I moved my “my_variant” folder there, did it finish the build without errors! Any thoughts why is that so?
BTW, the platformio docs about system directories mentions nothing about “variants_dir” that you used in your board.json file. How did you make it to search in your project directory?