SSL VPN -> ping to hostname only works again after reboot

Hi everyone,

I'm using SSL VPN between the Sophos Client and our XG 330 firewall.  Works for 95% of the people perfect but some people call me to mention problems with the network connection.  

On this clients: 

1.  VPN is still showing green light

2.  I can't ping hostnames anymore  (unable to resolve hostname) on the connected VPN network.  

3.  Nslookup of the hostname still works.

4.  Restart the VPN connection doesn't resolve the problem

5.  ipconfig /flushdns doesn't resolve the problem

6.  reinstall client + configuration doesn't resolve the problem

6.  Complete reboot of the Win10 machine resolves the problem for a certain amount of time. 

Anybody have a solution for this issue?  Thanks a lot in advance!