Membership plugin causing pageXOffset errors in IE

In IE (7,8,9) I get an error of "Error on Page" in the lower left corner of the taskbar.
The error reads:
'pageXOffset' is undefined

This error only occurs in my forum (simple:press) but it seems to be caused by the Membership plugin. That is, if I deactivate the membership plugin, no error code.

I realize this is a JS error, and according to simple:press, they only use the code on uploads. I get the error on pageload.

Any ideas on what might cause this?