Why can .svg be fixed by removing the line inkscape:connector-curvature?

Question

Adobe Illustrator

It is very interesting to know the answer to this question. For some reason, it happens that Illustrator does not save svg correctly and the file is broken. After that, it cannot be opened in Illustrator, and when opened in a browser, an error appears:

XML parsing error: prefix is ​​not associated with a namespace
Address: file:///Users/ilya/Dropbox/SVG_sep/buttons-30.svg
Line 6, character 1:<path id="path4204_25_" inkscape:connector-curvature="0" fill="#414141" d="M47.206,23.603c0,13.036-10.567,23.604-23.604,23.604

If you remove the substring " inkscape:connector-curvature="0"" — then everything will be fine both in Illustrator and in the browser.Inkscape I know is an open source vector graphics editor.Here is

an example file

Why .svg is not saved correctly? What does this line mean? Why does everything work without it?