Apache fails to proxy saying "AH01144: No protocol handler was valid for the URL"

Issue

• mod_cluster fails to proxy our traffic, returns a 500 response, and yields the following:

  [warn] proxy: No protocol handler was valid for the URL /foo. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.
  

• After completing our custom mod_cluster configuration, applications register correctly with mod_cluster_manager. However, when accessing the application we get status 500 response. In error_log we don't see any errors, only this warning:

  [Mon Aug 19 17:40:24.645391 2024] [proxy:warn] [pid 3530452:tid 3530548] [client 10.0.0.5:51965] AH01144: No protocol handler was valid for the URL /favicon.ico (scheme 'balancer'). If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule., referer: https://myserver:8080/myapp/