Hi! Just to be able to replicate your setup, as our initial tests didn't catch this one:
what packages did you have on jessie (I'm assuming squid3 + squid3-common from jessie, or did you have any other installed? The package did have the default config files or where they modified in some way? Did you have any config stuff left from previous squid 2.7 from wheezy around? Any other info you consider special on your setup? Regards... -- Manty/BestiaTester -> http://manty.net