Internet Connection for some clients but not all

[MessWeb]

Hi,
I've got FS running at a few sites but at one in particular I am getting loads of problems. Some of the clients can't login. They can see the network, connect to it, but then have no access to the internet where as otehr clients, sometimes in the rooms next door, get a fine connection with no problems. At some point it did work for them, as they have login details, but no longer.
This is also the site that always comes up with 'Apache Errors' all the time.

Thanking you in advance,

Andy