Thanks for the script -- I am not seeing any strange symbols either (tested under Windows, too), so I have no good idea what the underlying issue could be / why (some?) of your participants are seeing something else. One thing you *could* try is to explicitly specify the text that should appear on the "continue" button, by defining the /nextabel and /finishlabel attributes in your <survey>. Speculating a bit, perhaps it is a font issue, so you could specify a different font as well.
<survey check>/ nextlabel = "Continue >>"
/ finishlabel = "Continue >>"
/ navigationbuttonfontstyle = ("Arial", 2%)
/ pages = [1=questions]
/ responsefontstyle = ("Arial", 25)
/ itemfontstyle = ("Arial", 30)
/ txcolor = white
/ itemspacing = 5%
/ showpagenumbers = false
If you want to do the same for the instruction pages as a precaution, you can do that by adding an <instruct> element in the script:
/ finishlabel = "Press SPACE for next page"
/ nextlabel = "Press SPACE for next page"
/ prevlabel = "Press BACKSPACE for previous page"