This issue is caused when the referrer variable in your browser is blocked and unable to be read by the UBB software; this is an issue with a security application on your computer; it can be several things, including:
1. Internet Security Suites, such as Norton Internet Security.
2. Firewalls, such as Zone Alarm
3. Virus Scanners
4. You may be posting from a site which is not in your "allowed sites" list (multiple domains, viewing from a non-www variant of your forum, etc); you can check this through: CP -> Master Settings -> Primary Settings -> General Tab -> Advanced Options Dropdown -> Domains for HTTP Referer Check
5. Routers would be a possibility, you'd want to check over the security settings.

It's unrecommended to disable this security check, as it ensures any posts made are from your site, but it can be disabled by:
CP -> Master Settings -> Primary Settings -> General Tab -> Advanced Options Dropdown -> Disable HTTP Referrer Check

If you've determined item 4 isn't the issue, and you don't want to disable this security setting, then you'd want to go through your software applications and ensure any options labeled similar to "Block Referrer Values" are not selected.