Why does fencing fail on HP iLO 4 devices with firmware version 1.50 or earlier?

Issue

• Running fence_check in a cluster node where its fencing device is a HP iLO 4 running firmware 1.50 or earlier fails randomly:

# fence_check
fence_check run at Tue Aug 12 10:55:06 BST 2014 pid: 9335
Testing node1i method 1: FAILED
status node1i dev 0.0 agent fence_ilo4 result: status error
Testing node2i method 1: success
# 

• Running ipmitool manually to check the power status also fails:

# ipmitool -v -H <ip_address> -I lanplus -U <username> -P <password> chassis power status
Error in open session response message : illegal parameter

Error: Unable to establish IPMI v2 / RMCP+ session
Unable to get Chassis Power Status
#

• I can ping the iLO 4 IP address and I can login to the web interface and by SSH successfully using the same user configured for fencing in cluster.conf file.

• Resetting the iLO 4 (either by logging through SSH and running reset /map1 or through the web interface) solves the problem temporarily but the problem re-occurs after some time.