Oracle sessions randomly hanging behind fortigate

Hello everyone,

Hope someone could help us with this annoying issue

We put our oracle server (weblogic/DB) behind a fortigate. Fortigate is doing routing between our VLANs.

Computers from another network (so, where packets are routed by the fortigate), have their oracle session (TNS) randomly hanging. In oracle console, status of the client is : "SQL*Net message to client"

Fortigate policy have no filtering enable (any/any, no protocol/filter enable). We tried to create a dedicated policy where everything pass and increased policy timeout at 3600.

We also tried to delete TNS session helper.

Sessions are still randomly hanging

Do someone have an idea ?

Thank you