Telerik Reporting assemblies are built with AnyCPU target platform. Thus you can use x64 build configuration.
The problem in the case most probably is caused by:
- Referenced assembly targeting x86;
- Improper setting of the application pool. Test using local IIS and an application pool with turned off/on 'Enable 32-bit Applications'.
If you need further help, please open a support ticket and send us a demo project to check its settings, and details about the IIS settings.