Your feed URL ends in .html which is incorrect, and iTunes is not recognizing it as a feed as a result (you can't subscribe manually). I don't know why the Store is showing it - did you originally have .xml as the extension and change it to .html recently? - in that case the Store would be showing a cached version but might not update.
You can't just change the extension - that won't work. You will have to create a duplicate with the extension .xml, and then arrange to move the Store from using the current feed (if it is in fact using it) to the .xml version. Check in Podcasts Connect what URL you have registered - you can change it there and the Store will move to using the new feed; or use one of the other methods outlined here:
http://wilmut.uk/move