Ribbon - Unhandled access violation reading 0x0000 exception

Hi,

I have this Acad plugin (C#), where I simply create a Ribbon, Button on it that executes a Command.
Windows 7, AutoCAD 2013 all running x64.
Used the ObjectARX x64 Libraries for my references.
In my environment, everything works just fine, on my clients workstations... it shows 'Unhandled access violation reading 0x0000 exception'

Since client installed AutoCAD 2012 and then 2013, I tried a Repair, to no result.
Have verified all references, put all ObjectARX references, just to make sure, put debug lines in the code.
It crashes at first line:

Autodesk.Windows.RibbonControl ribbonControl = Autodesk.Windows.ComponentManager.Ribbon;

All other Commands seem to run just fine.
Any clue what could be the cause, what I could try more?