We have a 2830N (firmware 3.3.6.1_232201) which is in a business environment.
I have set up a single URL filter, on one word (facebook.com)
This is run on 2 schedules (00:00-12:00 & 13:00-23:59) to block facebook, but allow access over the dinner hour.
This all appears to work fine, and we get the expected blocked page during the normal hours.

However.

On one PC, an application to track vehicles is installed. This connects to the providers server via port 80 traffic.
it does not have facebook (or indeed.com) anywhere in the address it connects to.
I have run syslog, and can see that it passes sucessfully through the URL content filter, but will not connect.

If I however turn off the URL content filter, it works fine.

The only thing I can think of is that by going through the URL content filter, the packets are being changed so that the server rejects them.

I can browse to the website that the application uses fine, whether the filter is turned on or off.

Any ideas ?

If you tick the block access by IP address for the URL filter and that site is using a hardcoded IP rather than DNS for everything then that'll cause the site to break just like you're describing.