Try the following steps that typically helps with that type of error:
1. In MSP:
- Go to View > Macros > Visual Basic Editor
- Expand ProjectGlobal, Expand Modules
- Delete the modules that start with either CA_ and Niku_
- Expand Microsoft Project Objects, Double-click on Thisproject
- Select everything in the right panel and delete it, Save and exit VB
2. Go to your Control Panel->Programs and Features and just uninstall the CA Clarity Addin (you can leave the other 2 CA components)
3. Navigate to the following folder directory: CA\Clarity\CA Clarity PPM MSPInterface\addin and double click setup.exe to run through the install process for the Addin.
4. Then see if ou are able to open projects okay from PPM to MSP?