What error messages do you see
- on the workstation?
- in /var/log/httpd/error_log?
- in the dansguardian logs?
- in the iptables logs?
If you "ping" the "company website" from a workstation where it works, and from another where it doesn't work, do you get the same IP address (is there a DNS problem)?
If "ping" gives the same IP:
- does it succeed?
- does traceroute (or "tracert" on windows) to the "company website" indicate any problems?