Re: when login using captive portal on fortigate1000d then display ERR_EMPTY_RESPONSE
my approach, not necessarily the best one, would probably be ..
- test with different user (just to rule out that user is not failing to auth)
- test original user on another workstation, or via 'diag test auth..' on FGT, just to test user is OK
- use flow debug + diag debug app fnbamd 7 .. to see that auth is going OK and passing through expected policy
- check policies, there might be unexpected policy matching
Above mentioned tests should tell you a bit more and show the way where to go next.