MAC tx fail in Wi-SUN router based on mbed-os-example-mesh-minimal

[ljw60kr]

I am trying to construct a Wi-SUN with nanostack-border-router and mbed-os-example-mesh-minimal programs, but
I met an error message in a router based on mbed-os-example-mesh-minimal listed below.
I found a similar issue in "#288".
According to the issue, the problem was fixed in mbed-os 5.14.1.
However, I am facing a similar problem in mbed-os 5.15.4.
My HW and SW environments are as follows:
Border Router:
NUCLEO-F746ZG + X-NUCLEO-S2915A1 (S2LP)
SW: nanostack-border-router (mbed-os 5.15.4)
Router:
NUCLEO-F411RE + X-NUCLEO-S2915A1 (S2LP)
SW: mbed-os-example-minimal-mesh (mbed-os 5.15.4)

A part of log file.

[INFO][ksep]: Initial-key init
[INFO][wsps]: EAPOL target: 12:b9:d6:42:8d:18:e6:da
[INFO][wsps]: Initial EAPOL-Key trickle I: [360,720] 569, t: 557
[INFO][ksep]: Initial EAPOL-Key send, PMKID not set PTKID not set GTKL 0
[INFO][mMCp]: MAC tx fail
[INFO][wsps]: Initial EAPOL-Key TX failure, target: 12:b9:d6:42:8d:18:e6:da
[DBG ][secl]: prot timeout, state: 5
[INFO][ksep]: Initial-key finished
[INFO][wsbs]: received ADVERT Src:12:b9:d6:42:8d:18:e6:da panid:6ebc rssi:-50
[INFO][wsps]: initial key retry timer expired

[ciarmcom]

ARM Internal Ref: IOTCELL-2469