+++ This bug was initially created as a clone of Bug #1157766 +++
Please, read this description  and follow https://issues.jboss.org/browse/MODCLUSTER-430
This bug is deeply tricky and it has been around for quite some time. It's actually linked to Apache HTTP Server mod_proxy code integration.
We may even speculate whether it might be the root cause of some other mod_proxy integration issues, such as MODCLUSTER-391.
Actually JWS3 upgrade to httpd-2.4.x so the BZ should be gone there.
The issue is httpd 2.4/2.2 agnostic, so it will not be gone in JWS3.
I shall revisit the approach outlined in https://github.com/modcluster/mod_cluster/pull/119#issuecomment-58634743 in the beginning of February; I agree that the current PR is too much "mod_proxy dependent".
Make it a known issue... It seems there isn't a good fix :-(
http://httpd.apache.org/docs/current/mod/mod_proxy.html#balancerinherit might be used instead.
Michal could test if BalancerInherit does something with mod_cluster?
Ad comment 5: I remember I tried that back in the day (BalancerInherit). It didn't affect the error, because servers share the same pointer, i.e. the configuration is not merely copied through.
IMHO, There is a good solution: One needs to call the mod_proxy config function in mod_cluster config phase (instead the current implementation-fragile mimicking that function) See my question regarding the improvement in: https://github.com/modcluster/mod_cluster/pull/119#issuecomment-58634743
Resetting devel_ack flag, because it had been based on comment 3 that was obsoleted by comment 7.