Copy cost plan does not work on a Cost Plan

Document ID : KB000093087
Last Modified Date : 26/04/2018
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When I try to copy a cost plan, I get the below error:
"Error 500 Internal Server Error. This server could not retrieve the document due to server-configuration or other technical problems. Please contact your site  administrator".
If I try a different cost plan on a project, it works ok.
The cost plan 'Grouping Attributes' in this case was set to use Role attribute but one of the rows in the cost plan details did not have Role populated.
When you set an attribute as part of 'Grouping Attributes' then the attribute become a mandatory field which has to be populated.
For example when you attempt to manually add a row in the Cost Plan Details page, you will find that the 'Grouping Attributes' is a mandatory field
which needs to be filled in.