How to manage elasticsearch logs: disk almost full

Do you have some ideas about the meaning of this message ?

Unexpected character ('\' (code 92)): expected a valid value (number, String, array, object, 'true', 'false' or 'null') at [Source: (String)"\u0000Sw\xB1\u0000\u0000\u00061__BEGIN_JSON_

So, the problem is in the source host?
Thanks