Now I guess you got this message because the formula input you gave to the extension was either not valid LateX, or it became invalid because of a shell expansion which occured when the script was called by the extensions manager (this is a bug).
Actually I've tested it with the default input (so without $$).
Can you try with a simple "( \cos(x) )" LaTeX input?
Here the result :
pstoedit: version 3.44 / DLL interface 108 (build Jun 21 2006 - release build - g++ 4.1.2 20060613 (prerelease) (Ubuntu 4.1.1-2ubuntu3)) : Copyright (C) 1993 - 2006 Wolfgang Glunz Traceback (most recent call last): File "/home/menut/.local/share/inkscape/extensions/eqtexsvg.py", line 109, in ? e.affect() File "/home/menut/.local/share/inkscape/extensions/inkex.py", line 137, in affect self.effect() File "/home/menut/.local/share/inkscape/extensions/eqtexsvg.py", line 95, in effect svg_open(self, svg_file) File "/home/menut/.local/share/inkscape/extensions/eqtexsvg.py", line 46, in svg_open doc_width = float(self.document.documentElement.getAttribute('width')) ValueError: invalid literal for float(): 210mm
Jean-Luc