Looks like a problem with the reverse path check. Try to make a flow
debug:diag debug flow filter (f.e. destination port = dport; source
address = saddr etc.) diag debug flow show function-name enablediag
debug flow show console enablediag debug enab...
I would say the 3rd party company should support you. You can also try
using a newer version (5.2) of the FortiClient. You can find it here:
http://forticlient.com/
