El problema a ocurrido al actualizar el user agent? si es así prueba con las directivas de Firecold y comenta tu experiencia
acl youtube url_regex -i youtube.com
request_header_access User-Agent deny youtube
request_header_replace User-Agent Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
quedaria asi.
#========================== Squid 3.x Conf ===========================#
#
# Opciones de SQUID 3.x
#----------------------------------------------------------------------
http_port 3128 intercept
visible_hostname proxy.os.com
icp_port 0
#----------------------------------------------------------------------
acl youtube url_regex -i youtube.com
request_header_access User-Agent deny youtube
request_header_replace User-Agent Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
#----------------------------------------------------------------------
#request_header_access User-Agent deny all
#request_header_replace User-Agent Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
#----------------------------------------------------------------------
#error_directory /usr/share/squid3/errors/Spanish/
#----------------------------------------------------------------------
acl denegados url_regex -i "/etc/squid3/denegados.lst"
#----------------------------------------------------------------------