If you use NAT in the outgoing policy the FGT will use a different high-range port for each outgoing connection. It can only differentiate connections by the port number as it uses the same source IP all the time.
You can check " fixed-port" to force it to keep the source port number. But then, there will only be one connection possible per source port.
From the CLI Guide:
" config firewall policy
set fixedport enable
Enable to preserve packets’ source port number, which may otherwise be changed by a NAT policy. Some applications do not function correctly if the source port number is changed, and may require this option.
If fixedport is enable, you should usually also enable IP pools; if you do not configure an IP pool for the policy, only one connection can occur at a time for this port."
Ede Kernel panic: Aiee, killing interrupt handler!