BIG-IP 11.3.0 --> 11.6.0 Upgrade Failover Woes

Hello all,

During an upgrade of 11.3.0 to 11.6.0 I experienced issues when failing over to the standby device to upgrade the active. The devices are in an active-standby configuration, but not configured in an HA-group.

When forcing the traffic group on the active device to standby, the following was initially seen in the ltm log:

Oct 18 21:39:52 LB-F5-1 notice sod[5994]: 010c0044:5: Command:  go standby /Common/traffic-group-1 GUI.
Oct 18 21:39:52 LB-F5-1 notice sod[5994]: 010c0052:5: Standby for traffic group /Common/traffic-group-1.
Oct 18 21:39:52 LB-F5-1 err sod[5994]: 010c0035:3: Function run_external failed to fork at call to failover scripts.
Oct 18 21:39:52 LB-F5-1 notice sod[5994]: 010c0018:5: Standby
Oct 18 21:39:52 LB-F5-1 info tmm[25247]: 01190004:6: Resuming log processing at this invocation; held 32 messages.
Oct 18 21:39:52 LB-F5-1 warning tmm[25247]: 01190004:4: address conflict detected for X.X.X.X (XX:XX:XX:XX:XX:XX) on vlan $$$
Oct 18 21:39:52 LB-F5-1 warning tmm[25247]: 01190004:4: address conflict detected for X.X.X.X (XX:XX:XX:XX:XX:XX) on vlan $$$
Oct 18 21:39:52 LB-F5-1 warning tmm[25247]: 01190004:4: address conflict detected for X.X.X.X (XX:XX:XX:XX:XX:XX) on vlan $$$
Oct 18 21:39:52 LB-F5-1 warning tmm[25247]: 01190004:4: address conflict detected for X.X.X.X (XX:XX:XX:XX:XX:XX) on vlan $$$
Oct 18 21:39:53 LB-F5-1 warning tmm[25247]: 01190004:4: address conflict detected for X.X.X.X (XX:XX:XX:XX:XX:XX) on vlan $$$
Oct 18 21:39:53 LB-F5-1 info tmm[25247]: 01190004:6: Per-invocation log rate exceeded; throttling.
Oct 18 21:39:54 LB-F5-1 notice tmm1[25248]: 01010029:5: Clock advanced by 124 ticks
Oct 18 21:39:54 LB-F5-1 notice tmm[25247]: 01010029:5: Clock advanced by 136 ticks
Oct 18 21:40:13 LB-F5-1 notice logger: /usr/bin/tmipsecd --tmmcount 4  ==> /usr/bin/bigstart stop racoon

This resulted in a situation where both devices flipped back and forth from active to standby.

What I'm most intrigued about here is the following portion of the above snippet:

Oct 18 21:39:52 LB-F5-1 err sod[5994]: 010c0035:3: Function run_external failed to fork at call to failover scripts.

Can anyone shed some light on this error, and what it's implications might be?

Thanks! Chris