tested with PR, followed steps in Comment 4 and Comment 6 in bug 2040694, no error for http_config.follow_redirects, and no error logs from alertmanager container, we could see the configuration loaded in ${alertmanager_route}/#/status page
fix is in 4.9.0-0.nightly-2022-01-19-045232, based on Comment 1, set to verified
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (OpenShift Container Platform 4.9.17 bug fix update), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2022:0195