Solved this problem a few minutes ago for my installation.
In my setup i have Pfsense 2.3.2, squid and squidguard.
The problem i've dealth with, was the certification error "http". After clicking everything possible in squid configuration i've found out it was the squidguard common ACL "blk_BL_adv"
I imagine that many users use the shallalist blacklist, at the very moment i disabled that rule everything in the Man in The Middle worked like charm.
I'm not a programmer nor a squid expert, if anyone in this forum can contact the squidguard developers maybe they will find out if i was lucky or if there's a problem with squidguard, shallalist and ssl filtering
Sorry for my poor english.
Bye