Error in joining SPS cluster to SPP cluster (Safeguard 6.11)

Hi Safeguard Experts,

We hit an issue when trying to link SPS cluster (2 SPS nodes) to SPP cluster (3 SPP nodes) and it returns "Internal Server Error: The server is not available". We then searched the log from SPS and it does not show anything obvious. It is working in our dev environment where there is only 1 SPP and 1 SPS, but now it's not working when we have clustering in SPP.

SPS Log:

restserver[928]: Starting; action='Joining to SPP', spp='{SPP_IP_ADDRESS}', spp_ip='{SPP_IP_ADDRESS}'
restserver[928]: SPP public certificate chain sha1 fingerprints; spp_cert_chain=['{cert_thumbprint}']
restserver[928]: Starting; action='SPP join request', spp='{SPP_IP_ADDRESS}', spp_ip='{SPP_IP_ADDRESS}'
restserver[928]: SPS server certificate sha1 fingerprints; issuer_chain=['{issuer_cert_thumbprint}']
restserver[928]: Finishing; action='SPP join request', spp='{SPP_IP_ADDRESS}', spp_ip='{SPP_IP_ADDRESS}'
restserver[928]: Finishing; action='Joining to SPP', spp='{SPP_IP_ADDRESS}', spp_ip='{SPP_IP_ADDRESS}'

We have already followed the pre-requisites and the procedures from the administrative guide

1. Link SPP from the SPS Central management node, with configuration sync enabled (configured Managed host role on the minion node)

2. The SPP user used in the link operation is having Appliance and Operation permission

3. Verified there is no SPS policy that contain safeguard_default

4. Tested telnet connection on port 443 and 8649 and it is working in both directions

Any ideas or suggestions?

Thank you