Seems like Inkscape's rendering of some of the new features is still buggy.
In the title where it says "Inkscape", those square like shapes are artifacts. Although I do think the effect looks cool, probably will be gone with a release candidate or the stable one.
I've seen similar issues with artifacts also. I can't remember if there's trick in Inkscape to work around it, but I'm pretty sure that you can use Batik Rasterizer as a fall back if you need to.
This was rendered over three years ago, I really hope the issues that make this squares arise have been corrected by now. I recall the gaussian filter was a recently added feature in this version. Really cool to mess with it.
It is maybe a bit too dark (?) (a slightly stronger glow behind the text could do it).