All logging formats have been removed from the core and records.config, and are now exclusively enabled and managed via logs_xml.config. The default file will create the binary squid blog as before, but if you have made changes to your records.config prior to upgrading, you must apply those changes to the appropriate configurations in the XML configuration.
SSL host verification
As of TS-3608, ATS will now correctly verify that the host header and certificate SN matches properly. This can cause problems, if your setup is such that you have a mismatch, and just didn't notice it before. If that is the case, you can either fix the certificate, or possible tweak the proxy.config.url_remap.pristine_host_hdr configuration.
The cache format in this release is compatible with previous 4.x and 5.x releases.