Below is a good step by step troubleshooting process for sites being blocked for a reason other than Web/URL content filtering.
• Turn off security and try again. Can they now access the website without an issue?
• Get the URL or IP address they are attempting to access.
• Go into OmniNet's MDS Manager > Login > Select Cloud Link > Advanced tab > Security Bypass Rule
• Select Add New Rule
• If entering an URL address: Type By: URL > add both the https:// and http:// formats for the website. SAVE.
• If entering an IP address Type By: IP > add IP address and Network Mask. SAVE.
• If the issue still persists. Submit a ticket with details on the source problem machine (Source IP and network info) - This is especially important if they are not using MDS DNS.
*If you are unsure what the Network Mask is, go to (http://www.subnet-calculator.com/cidr.php) Enter IP address > Mask Bit and it will calculate the CIDR Netmask.