What is wrong with my file name?

I copied this web page from here to a Windows machine then I renamed it from SQLSaturday #588 - New York City 2017 Sessions Schedule.htm to SQLSaturday #588 - New York City 2017 Sessions.html. I edited the page to remove some stuff like the download buttons and it loaded fine on my machine.

I then uploaded it to my GitHub account. Now it is broke and I do not know why. It 404s on this:

http://ift.tt/2qkqXEx - New York City 2017 Sessions.html

I do not know why it puts a %20 for one single space occurrence and not the rest. Doesn't matter though since if I replace the %20 with a space or all the spaces with %20 it fails just the same. If I rename the file to x.html it works.

Anyone know why I can not use the file name and what I can do to fix it? This is my page. You need to click Events then SQL Saturday so there will be two links, SQL Saturday 588, NYC, May 20, 2017. One works and the other is broke.