Fortigate SSLWeb: accessing subfolders fail thru nextcloud web

Hello
we are accessing nextcloud web interface thru fortigate ssl web vpn page, everything works well except cannot access subfolders,
when i check subfolder url is duplicate like below;

https://xxx.xxx.xxx/proxy/51bba872/https/cloud.yyy.yyy/index.php/apps/files?dir=https%3A//cloud.yyy.yyy/remote.php/dav/files/nextcloud/Photos

any ideas?