I ran into this error yesterday when I was using the Aggregation Design Wizard in Analysis Services 2005.  I was finally able trace the problem to a conflict between the Aggregation Design Wizard and a custom assembly that I had in my project.  I still don’t know why the custom assembly was interfering with the creation of aggregations, but here is how I resolved the issue:

1)     Remove the custom assembly from your project in Business Intelligence Development Studio.

2)     Deploy your project.

3)     Design the aggregations for your partitions with the Aggregation Design Wizard.

4)     Add your custom assembly back into your project in Business Intelligence Development Studio.

5)     Deploy your project.