I subbed to it on my own TT-RSS installation (obviously removing the spaces) and it worked just fine. Before that, I put it through myfeedsucks and it didn’t seem to have a problem with it either.
I’m going to guess this is a certificate error of some sort; make sure your operating system is up to date. You can also SSH into the server and try getting the feed using curl or wget, that should give you a bit more of an idea why things aren’t working.
add feed via batch add (preferences → feeds) and then use feed debugger on it
that would be a libssl error, URL validation is just syntax. ports, scheme, etc.
still not a bad idea to use wget in case his IP is blacklisted and it redirects to localhost or something, there are weird people who actually do stuff like that.