Does the ribbon start out grayed out when you first open ACA, or does it happen some time later, after using the program? Do you have both the ACA and ACAD CUIX files loaded at the same time? If so, there is a known conflict that arises due to the fact that both CUIX files contain contextual ribbon tabs for the same object type. One frequently encountered conflict is when selecting MTEXT, as both have a contextual ribbon tab triggered by that selection.
The simple solution is to not load both at the same time. If there are features in the ACAD.cuix file that you must have, then you will want to either edit the ACAD.cuix file to remove the contextual ribbon tabs that it has in common with the ACA.cuix (archive a backup copy of the out-of-the-box file first) or create a custom CUIX file and transfer the things you want from the ACAD.cuix file to the custom CUIX file, omitting any conflicting contextual ribbon tab.
David Koch
AutoCAD Architecture and Revit User
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