Totally new to Inkscape and am using .47 on Mac OS X 10.6.4.
I was previously using .svg files downloaded from WikiMedia Commons, opening them in GraphicConverter as large .PNGs, and then editing them there (recoloring, etc.).
One had a slimmer outline than I would have liked, and I got the bright idea, "Hey, why don't I open it in that Inkscape I've heard about and edit it there?" I open up the .svg, Select All, and used the very first filter in the list, Black Outline. It did exactly what I wanted and looked great! So I Save As an Inkscape .svg.
But the resulting document is only an error message, which looks like this:
"This page contains the following errors:
error on line 100 at column 5: Encoding error
Below is a rendering of the page up to the first error." [Below is actually nothing. None of my software can do anything else with this file.]
So I tried saving it as a regular .svg instead of the Inkscape flavor. This resulted only in what appeared to be a clone of the original .svg file, minus the changes made by applying the filter.

There is a box around the image (before I select it). When I use the filter, the additional black outline added by the filter went beyond the edges of the box. "Perhaps this is the problem," I thought, and so I tried scaling the filtered image down with Transform:Scale so that it fit within the box, then saving it as an .svg. This failed in the exact same way detailed above.
As a beginner to this program, I expect to have a lot of what it's doing go over my head, but I figured applying only one filter and saving to the same format would be relatively straightforward. There must be something utterly basic I am missing. I did examine the manual; the description of filters indeed makes their use sound very straightforward, and I could find no reference to this particular error message in the free online version of the manual.
Can anyone tell me what's causing the errors? I welcome suggestions on how to proceed with my goal of an error-free black-outlined .SVG. Thank you in advance for your time.