Strange VPN-Tunnel problem, Smooth+FEC Tunnel works but just bonding not...

So, strange problem here with a Balance 310 5G. Connected to a FusionHub.

Balance Router on FW 8.1.3 build 5162
FusionHub on FW 8.1.3 build 5023

Two profiles for the SpeedFusion connection:

  1. Smooth+FEC (WanSmoothing=normal, ForwardErrorCorrection=low)
  2. JustBonding (WanSmoothing=off, ForwardErrorCorrection=off)

I get the following situation for the WAN connection, this i the first time i see something like that. WAN is working. The Smooth+FEC profile is working, but the WAN for the JustBonding profile is not. Really strange. We are having a pop up live-studio for streaming for a customer, WAN is provided by them. But what can cause such an error? Strange port issues maybe for the 2nd profile?

The error logs just saying:

SpeedFusion: FusionHubWien (FusionHubVM-Wien, sn:xxxx-xxxx-xxxx) disconnected from FH-VIE (2 - JustBonding) (link failure detected)

The “JustBonding” connection to our other FusionHub (FH-FRA) is working normal.

each subtunnel consumes another data port. so 4500, 4501, 4502 etc
So it looks like a firewall / port related issue to me. Check your fusionhub confiig that ports 4500-4505 are allowed.

There is no firewall active on the FusionHub side. It may be an outgoing firewall issue on the customer location. You can change the standard ports for that right?

When i delete the 2nd subtunnel and change the first to also do just bonding all works normal. So it could be that outgoing port 4501 is blocked on the customer LAN we’re currently at.