I finally managed to get squid with kerberos authentication and LDAP group checking to work. I am using squid 4.1, kerberos v5 and Linux Mint Cinnamon 20.1.
Everything works fine, but I got a little problem: As soon as I am using Kerberos as authentication method, squid won't write the access.log anymore. When I use NTLM, squid is writing the access.log perfectly. But with Kerberos it won't work. Does anyone know, why this is?