Bueno, al final era una violación de una regla de Mod_security para el dominio, por parte del proveedor del hosting...
Contacté con ellos y solucionado, os dejo el código de error:
Código:
Time: Mon Dec 10 11:34:20 2018 +0100
IP: 213.143.50.112 (ES/Spain/-)
Failures: 5 (mod_security)
Interval: 300 seconds
Blocked: Temporary Block for 36000 seconds [LF_MODSEC]
Log entries:
[Mon Dec 10 11:33:04 2018] [error] [client 213.143.50.112] ModSecurity: Access denied with code 406 (phase 2). Pattern match "(?:\\b(?:(?:type\\b\\W?\\b(?:text\\b\\W ?\\b(?:j(?:ava)?|ecma|vb)|application\\b\\W*?\\bx-(?:java|vb))script|c(?:opyparentfolder|reatetextrange)|get(?:special|parent)folder|iframe\\b.{0,100}?\\bsrc)\\b|on(?:(?:mo(?:use(?:o(?:ver|ut)|down|move|up)|ve)| ..." at REQUEST_FILENAME. [file "/usr/local/apache/conf/modsec2.user.conf"] [line "140"] [id "1234123404"] [msg "Cross-site Scripting (XSS) Attack"] [data ".cookie"] [severity "CRITICAL"] [tag "WEB_ATTACK/XSS"] [hostname "www.dominio.com"] [uri "/js/jquery.cookie.js"] [unique_id "XA5A4LIhp6MAACHaYiEAAAAJ"]