Troubleshooting the "Invalid JSON Error" in WordPress involves several steps. Firstly, check for SSL and mixed content errors. Secondly, consider disabling the block editor and switching back to the classic editor. Next, look for any conflicts with the WordPress REST API. Finally, adjust the WordPress permalink structure and update the '.htaccess' file. Remember to back up your site before making any changes, and always test it after each step to see if the error has been resolved.