The framework requires all json files to be valid, while the game seems to tolerate some minor errors. This has nothing to do with the complexity of a contract.
As displayed in the error, the issue is a wrong character following a '\' character in line 172 of a json file. From the rest of the error, it can be derived that the json file in question is a contract file, so the culipit is in the contracts subfolder.
As you see, the framework just prints errors that occur without giving you the filename, so it could be any contract. Try isolating each one to see which one fails, or edit __init__.py by adding print(contract_definition) before line 121 and look at the last filename above the error.
I will try to remember to add support for displaying the filename when such errors occur in future versions. No promises though!