Upgrading my 2860 from FW 3.7.8.3 to 3.8.1 or 3.8.2 breaks DNS lookup when configuring firewall rules with a DNS Filter.
Both when using default DNS Filter or any custom DNS Filter.

Symptoms are DNS timeouts on all clients.

Removing DNS filter from firewall rules and specifying same URL and Web Content filters directly resolves the issue.

Setup is VirginMedia SH2ac in modem mode to WAN2 on the 2860.

But good thing with DNS filtering (URL and Web Content Filtering) is that all HTTPS requests are filtered, so for now have reverted to FW 3.7.8.3

I'm having the same problem with DNS filtering on a 2860. Funny think though it was working ok up until yesterday and nothing has changed with my config, im still on the same firmware I was the day before and settings haven't been changed (except to stop dns filtering)

Download the Draytek Router Tools which include a syslog application.
Configure your router to forward syslog to the PC you install the syslog application on.
Under CSM ensure you enable logging.

On occasions, I can see from my syslog that the CSM server sometimes becomes unreachable and logs "query timeout" notification, this will cause delays and timeouts when browsing the internet.

Thanks eric.jansen I will give that a go and see how I get on.