I have had exactly the same problem with the exact same version of actinic.
I ended up with either: 'Quantity: The maximum quantity allowed to be ordered for this product is 1.' or 'Quantity: The quantity must be a whole number between 1 and 32767.' doing exactly what you have tried and others have mentioned here.
In the end all I did was goto Design ¦ Text ¦ Misc
In the list is an entry called Quantity Prompt. Although this had been set up with a default of 'Quantity' I removed it and re-entered it. Voila, it all sprang to life.
This may or may not help you, but anythings worth a try.
Got it working, was an issue with the names of the quantity text box and the quantity hidden field. The productID variable was followed by a 2, which meant the productID was always wrong when I tried to add to cart.
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