One thing you can check is open a browser and type in the following:
where x.x.x.x is your controller’s IP address, and yyyy is the md5 hash of your device password (you can use https://www.md5hashgenerator.com/ to generate md5 hash from your password).
This should return controller settings in json format. One possibility is that the json string is corrupted, like there are garbage characters in the string. You can copy the entire string and paste it to a json parser (such as http://json.parser.online.fr/) to check its validity.
If indeed the string is corrupted, you can perform a factory reset. But I do suggest you keep a copy of the output of the /ja command above, as that will help recover the settings after the factory reset.