Problem

Video doesn't play in Digital Acquire when Previewing or Recording.

H.264 doesn't play in Internet Explorer in the Live View for a camera.

Solution

Make sure that ActiveX isn't disabled for the machine.

Check Internet Explorer->Internet Options->Security->Custom level...->ActiveX Controls and plug-ins
Check group policy: gpedit.msc->Local Computer Policy->Computer Configuration->Administrative Templates->Windows Components->Internet Explorer->there are multiple ActiveX related policies

Make sure that either a proxy isn't configured or that it one is that it allows streaming from the cameras.

Check Internet Explorer->Internet Options->Connections->LAN settings->Proxy server
Check (on domain controller) Group Policy Management Editor->User Configuration->Policies->Windows Settings->Internet Explorer Maintenance->Connection->Proxy Settings

Any policy changes made locally (on the workstation) will be lost after a reboot. Therefore they must be changed centrally on the server.