Ah, I found the file you posted. All the important things you didn't mention are also forbidden.
With my copy of Acrobat (not 10.1.10) I cannot change the watermark.
Perhaps you can post the file from the SAME original (important) but with the watermark changed. We might be able to see what has happened to the file.
If someone else has 10.1.10 perhaps they can try to change the watermark.