Fortigate UTM + XGS2230 Stack Nebula not work

tczauderna

I have a problem

I have already reported it to support but while waiting for an answer maybe someone already knows the solution.
Namely I have a set of UTM Fortigate + 2x XGS2230 switches connected in a stack.
And I wanted to set LACP between UTM and the switch stack in ports 1/48 2/48.
The switches are connected to nebula.
Unfortunately fortigate keeps changing LACP down. LACP with Synology works properly. That's why I'm asking on the forum how to set it up, probably mainly fortigate because you can see that nebula is limited :(
Configuration scheme for fortigate, the basic one

set vdom "root"

set type aggregate

set member "port1" "port2"

set description 'lacp_example'

set lacp-mode active

set lacp-ha-secondary disable

set lacp-speed slow

set algorithm L4

I tried to lower the algorithm to L2 but it didn't work. I'm still doing 2 iterations with lacp-speed

PCAP on fortigate doesn't return anything interesting but I'll try to squeeze something more