This is odd but could be connected to your particular version of Mac OS X Leopard as QCAD 2.2.3 was compiled using Snow Leopard. Could you please try QCAD 2.2.2 instead (compiled on Leopard):
Mac OS X 10.5, 10.6 (Leopard and Snow Leopard)
Universal Binary for PPC and Intel
The package is available from your download area:
* Visit our Online Store at https://www.ribbonsoft.com/store
* Click on "My Account".
* Login with your e-mail address and password you have chosen when purchasing QCAD (or click on the link "Forgot your password?" if you don't remember your password).
* Click on the "View" button beside the appropriate order.
* Click on the "Download" button beside the appropriate product.
* Your download area is shown and you can download the latest version of QCAD Professional for your operating system.
* The packages you download from your download area are fully functional and don't require registration or activation of any sort.