This issue has been solved. I upgraded to the latest version of the IBM Informix SDK (from version 3.70 to version 4.1) and it worked after that.
I'm not 100% sure if it was the upgrade itself, or a change I was forced to do the Path on my PC in order to do the upgrade. The path change was to add "C:\Windows\SysWOW64" to the start of the Path (it was already in the Path, but not at the start).
Thanks also to Don for his assistance with this issue...