Invalid namespace URI when converting certain CHM files

I know .chm files are sometimes a little finicky; I've gotten most of mine converted to epub, but a number of them will not convert to any format (including .html via calibre). They all have the same error:


Is there any way to fix this, i.e. ignoring this specific line in the conversion?

I've converted to HTML with 3rd party software, but this creates a directory with one html and lots of .mht files; I'm not sure how to use those to make an epub.