No, I don't think it's a problem of the server, but the IDE itself... I haven't fired-up the IDE at all today (Why not? Breakfast with my parents, and other things in life.) IF I were to shut-down the WiFi modem, and fire-up the IDE, it would come up set for the UNO, and see no active Com port. If I click on board, it will only show Arduino boards.. But, the instant I turn on the modem, and restart the IDE, it will request any updates to the libraries. If none, or the same, it'll NOW add the ESP-8266 selections to the board list. If I were to select another board (arduino), it would still retain the 8266 selections at the bottom. Okay, turn off the modem again, close the IDE, wait a few minutes, start the IDE again, it would check for the JSON file, and fail.. Go to Tools > Boards, and........ Where did it go? the list ends at Arduino UNO WiFi, and a white down-arrow.. the ESP-8266 Modules list is gone!... Sorry to say, it Does look like a problem with the boards manager maintaining 'other' boards, whether the json is read online or not.