If they have the domain in their IE Full Trust settings, that could be a problem. Have them check Full Trust sites in Internet Explorer's Tools->Settings->Security tab.
If the form has .NET code and they have installed a .NET app that modified CAS, that could also explain the situation. In that case you will have to open CAS and check.
1. Open Control Panel and click on Administrative Tools
2. Double click Microsoft .NET Framework 2.0 Configuration
3. Expand My Computer
4. Expand Runtime Security Policy
5. Compare everything under that tree with a user who has it working (this is a really slow process, so hopefully it's the first issue above)...
Other possibilities:
- Not authenticated to the domain (accessing from a cross-site)
- InfoPath form tries to load another site that they don't have access to
- etc.
Patrick Halstead
Project Manager at Qdabra