Problem is Browser object. Delete it and do something else in it and it will work. It seams, browser object cannot convert number type to string type (probably a bug in browser plugin).
You may try to create a text object in this capx and replace the "Browser-Log" by "Text-set text",
print the instance variable by text, will get the same error message.
On the other hand, if you try to print other expressions from this overlapped sprite; eg. Animationframe, angle, etc, it will work well. It looks like only instance variable hurts.