Guido,
I may be telling you something you already know, but in the name of being thorough let me offer the following.
I no longer have a P800, but I am using a P5000 and I "think" I remember the driver settings being more or less the same.
I routinely print 17x25" on a Windows 10 machine, no problems.
One thing I used to routinely get confused about is the "height" vs "width" terminology in the driver settings for User Defined sizes. Is there a possibility that you're reversing input on the numbers defining your paper size? See the illustration below for how I have mine set up:
I apologize if I'm just adding noise to your issue.
Rand