[kronolith] Event modification are not synced to remote device using CALdav if horde cache enabled

[Giorgio Paolucci]

Hello everybody,
I have a horde webmail installation 5.2.11 on a clustered debian wheezy
platform.
Horde is installed on a 4 node cluster (2 clusters of 2) and clients are
balanced on a round robin DNS basis; to keep sessions sticky, each node is
configured with an apache reverse proxy that forwards (based on cookie
values) requests to apache2 backend instances listening on non standard
ports on each node. Php caching system APC is installed.

If I enable horde cache (local file system on each node), users who
subscribe calendars through CALdav don't receive updates on their
smartphones, or better:  they receive new events or deletion of events,
but not modifications of existing events made through horde web interface.
Apache logs show that caldav clients sessions are sticky and each smarphone
query always the same node of the cluster.

With horde cache disabled, everything works fine. Any clues to investigate
this issue?

Thank you in advance.
Best regards.
Giorgio