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Yeah, I've got the same message quite some times and it almost drove me nuts.

First of all, make sure you don't have an old config where the "I2CAddress" setting is set to 0x88. The "0x" is not being recognized by the parser. The newer configs have that setting written in decimal notation.

Also, make sure that the file size doesn't exceed 1200 bytes (remove the materials if this is the case).
A good way to test it is to copy and paste the config file in here: https://arduinojson.org/v6/assistant/
That's a tool from the maker of ArduinoJson, which parses the JSON string and check its size and syntax.

If that doesn't solve it, try using a different SD-Card, which is what I had to do once in another project using the i3 mini controller.

Another thing I've discovered once is that the free memory dropped to about 200 bytes (which usually is supposed to be about 2000 bytes). This happens if the RX line of the 2nd serial port is open or disconnected (the one that goes to the SMuFF-Ifc). Either connect it or use a pull-up resistor on that input pin.