The first serialization succeeds, why do the next ones fail?
CAUTION: SLIPPERY FLOOR
The Arduino Library Manager installs the ArduinoJson version 6 by default.However, using version 5 is highly recommended because version 6 is still in beta stage.
Open the Arduino Library Manager and make sure that ArduinoJson version 5 is installed.
This is usually caused by a reused JsonBuffer.
The solution is simply to NOT reuse the JsonBuffer.