Today i faced same problem, v5.6.0 build1449 (GA), Windows 10 PRO 64bit (7 Pro), FortiClient 5.4.3.0870
Some of VPN users cannot connect to SSL VPN, getting "Unable to establish the vpn connection. The vpn server may be unreachable".
SSL VPN tunnel was working fine from the start 2-3 month stable.
I've got ident. PC with same configuration and connecting to tunnel without any problems. (Internet Explorer settings ident.)
Turn Off Firewall
Turn Off Antivirus
Restore Fortigate backup to date when all was working, but i have same error, so i guess its not Fortigate side.