Hello @tedstur,
First I think you have the feed address wrong. It should be https://missionexus.org/feed/podcast/.
The specific iTunes error is that iTunes cannot read your feed. I tried a few tests and did not see an issue. I then tried the http version of your podcast feed and it gest further in the validation on iTunes. https://missionexus.org/feed/podcast/. The error now is that your image. I did a quick look at the image and it appears to be a PNG in CMYK. Please convert the CMYK to RGB to solve that issue. Usually saving the image as a jpg automatically fixes this.
As for the https version of your feed, can you tell me more how you have the SSL setup? I did a quick test with ssllabs.com and though your SSL is setup wonderfully, the one problem it does return an “SNI unrecognized_name” warning. This can be fixed by matching the host name to the entries in Apache’s ServerName or ServerAlias settings. This SNI (server name identification) needs to work correctly for iTunes since iTunes uses Java (code base has a problem with the SNI unrecognized_name warning.