In /etc/pulp/settings.py check the CACHE_ENABLED setting. After upgrading to 4.5-RC2, I had the same redis issue. CACHE_ENABLED was set to False. There’s a note in the pulp documentation stating it should be set to True, however.
I changed the setting to True and restarted the pulpcore services. After that, repo syncing starting working again.
After around digging in foreman-installer, I found that pulpcore_cache_enabled was set to false in /etc/foreman-installer/scenarios.d/foreman-answers.yaml. I changed it to true and ran foreman-installer again and got the expected result ("CACHE_ENABLED = True") in /etc/pulp/settings.py.