This error occurred from someone scanning my websites for vulnerabilities, not form normal usage.
\xC0\xAE and \xC0\xAF were used, which is a unicode dot and forward slash.
ErrorException: json_encode(): Invalid UTF-8 sequence in argument - library/XenForo/ViewRenderer/Json.php:179
An example error can be made via css.php:
This particular unicode seems to cause errors with JSON anywhere it's used, though not always server errors.
Unicode such as Japanese doesn't cause an error: