SSLVPN disconnecting

Hello,

SSLVPN configurations were working good until yesterday. There is no changes in configuration. The Fortigate firmware version is 5.2.6

The some clients have Windows 7, Windows 8 or using FCT 4(SSLVPN CLIENT), FCT 5.2.4 or FCT 5.4 but all of them have this issue.

They see that vpn stops %98 or connecting-disconnecting.

Is it bug?

Thanks.