On the Challenger Viewer on Android didn't like it when I set it to show folders. First folder was like: "http", the next was ":" and so on, terrible to browse using this. And by normal use, it would show every issue that started with "01" or "00" etc. Which isn't ideally either, this might be an issue with the app itself though.. Not sure.

This happen to me the other day as well, canyouseeme.org said the port was opened and server answered, but I couldn't use the URL locally or over the internet. I'm using Windows 8.1. Restarted the server and it worked again,it was unusual.