Can you check if you have everything properly configured as in the following admin guide:
+ Check if you have the correct NAT rule, security rules.
+ Check if the application host can be resolved by DNS, to confirm this, run the following command via command line:
> show system setting ssl-decrypt dns-cache
+ If the issue still persists, I would suggest upgrading Clientless VPN to the latest software, this can be done from Device> Dynamic Updates> Check Now to see the latest updates.
I hope this helps. Let us know if you are still experiencing any issues.