Forticlient report to user the vpn server is unreachable, but FortiAuthenticator says wron

Dear all,

We use Fortigate + FortiAuthenticator for SSL VPN 2FA for remote access users.

Some users reported that Forticlient reported the server is not available.

Then we based on the user AD credential to trace the user log on FAC.

And we find FAC reports Wrong LDAP password at that moment.

We also test out the Fortigate connection and confirm no any issue.

Why the Forticlient reported VPN server not available but not wrong user name and password?

Is there any solution also?

Thanks. 

I try to open a TAC case for this issue, but he finally lets me open post here and asks for help.