ssh push to secondary Geo instance broken after update to 17.10.8 with: undefined method `geo_custom_ssh_action' for EE::Gitlab::GeoGitAccess:Module

I'm maintaining a Premium GitLab instance with 350 users. We are running a Geo setup, and after updating to 17.10.8 an ssh push to the secondary fails with "Internal API error". After further investigation, it's the internal/allowed API that fails, and exceptions.log shows the following:

undefined method `geo_custom_ssh_action' for EE::Gitlab::GeoGitAccess:Module"

I can post the full backtrace if necessary, but it's very long. Otherwise, instance reports to be healthy and all services are running, and pulling works fine. Pushing to the primary instance also works without problems.