We recently found a possible bug in the app when trying to test the feed: If you remove the feed and try to re-add it, the app will return an error that the URL can not be found. The only solution we found to getting the app to accept the URL again is to reset the phone. The app seems to be retaining some bad data under certain circumstances and not clearing it out. This does not happen in all instances, but we were able to duplicate this on two different iPhones. Our IT team has reached out to Apple to see if they know why this may be happening and if there is a work around.
It is worth noting that in most cases you should not encounter this error.
However, if you are getting an error in the Apple Podcasts app that says the URL can not be found, this may be the reason. In that case, you can use another app like Overcast (free) or Downcast (paid). Both support adding subscription-based feeds.