In SharePoint 2010 designer there is a concept as Globally reusable workflow, through which you can copy and modify and reuse a globally reusable workflow. If you are trying to copy and modify any reusable workflow in a sub site, you may get an error saying:
The workflow is read-only. Would you like to make a copy of this workflow to open or edit? like below:
And if you click on Yes then you will get a dialog box saying like below:
The server could not complete your request.
For more specific information, click the Details button.
And when you will click on the details button you will get an empty box.
The solution to this problem is do the same step at the root site of the site collection. You need to open the root site of the site collection and Copy and modify the globally reusable workflow.