Website issue with SSL-VPN

Hello,

we have a strange problem. We configured a ssl-vpn web-only portal with a bookmark to a http website.

This website is our ITSM tool (www.topdesk.com) which we want to have access for our external partner. So the login works fine on the ITSM tool but after then we have problems to use this tool. Some buttons doesn't work or you can't use a text field.

I'm not sure if this is a problem of our Fortigate or the website of the ITSM tool.

We tried 3 browser (IE11, FF, and Chrome) and all browser have difference issue.

We have a Fortigate 500D with SW v5.6.6 build1630.

Do you know this problems or have you any idea how we can solve this.