Customized workspace/ribbon won’t save

If a user makes changes to the Ribbon or to Workspaces those changes are not being saved after quitting out of Civil 3D and relaunching the software.

Solution:

The user makes the CUI ribbon or workspace changes that they want in Civil 3D and then saves those changes. Leave Civil 3D running.
In Windows Explorer navigate to the C3D.cuix, right click on it, click Properties, and on the General tab checkmark Read-Only. The file can be found at C:\Users\<<username>>\AppData\Roaming\Autodesk\C3D 2016 or C3D 2018\enu\Support
Close Civil 3D
Remove the Read-Only protection of C3D.cuix – right click on it, click Properties, and on the General tab UNcheckmark Read-Only.
Start Civil 3D