The problem is that the cql_filter parameter has an special character:
CQL_FILTER=etiqueta like u&'Garaf\00EDa'
As a result Geoserver throws the Exception you can see in the attached file:
Could not parse CQL filter list
Caused by: org.geotools.filter.text.cql2.CQLException: Encountered "like u" at line 1, column 10.
standard_conforming_string is on in postgres.
If we execute the same query in postgres it returns a valid result.
select * from "limites_municipales" where etiqueta like u&'Garaf\00EDa';