I have found a partial solution to this problem (which is caused by the OCR Engine integration into Nitro Pro).
Go to "File" -> "Preferences" -> "Conversion" -> "Advanced text recovery" and untick "Automatically OCR image based documents before conversion" at the bottom.
Seems to work for both conversion of images as well as direct scanning into Nitro Pro but does (on my PC) cause "Not responding" error messages during the conversion process.
Not an entirely satisfactory outcome, so I will be looking for a refund on this software.