This usually means that the user’s machine doesn’t have a sufficient graphics card/GPU to run 3D graphics apps in the browser. Often times we’ll see this when the user is trying to access HOOPS Communicator from a VM or when they are using Microsoft Remote Desktop to a machine. RDP disables 3D acceleration (in some cases it seems not to) so it causes issues. A good test is to ask the user to try this link and see if it works: http://webglsamples.org/aquarium/aquarium.html
This will help you determine if the problem is HOOPS Communicator specific or not.