Cisco AnyConnect dropped and can't reconnect without reboot

I am using Cisco AnyConnect on Ubuntu 18.04. The VPN sometimes got disconnected unexectedly. I try to reconnect, even quit the GUI, kill all process for programs under /opt/cisco, restart NetworkManger service. No use, it still can't initiate the connection (no login dialog). What am I missing? I guess there is something with the route is not cleaned up.

Update:

I got also this message from VPNGUI after a while:

The VPN connection failed due to unsuccessful domain name resolution

This article explains something about external DNS. I cleaned that up, and checked DNS information in Ehternet setting. Still couldn't get rid of the error.

Update 2:

This is the command that worked for me at last:

systemctl restart systemd-resolved.service

Solution 1:

I was getting crazy with that problem. Update 2 works perfect!!!

systemctl restart systemd-resolved.service 

Thank you :)