I am getting the same icon that you are getting whether I use mobile Safari or iCab Mobile identified as Firefox or even if I identity it as mobile Safari. I wonder if that means perhaps the podcasts was not created correctly. However, I know absolutely nothing about creating a podcast.
You obviously have been doing this successfully up until this point, but by virtue of the fact that I am getting the "No Plugin" icon in both browsers - whether viewing as a desktop browser or a mobile browser, I simply assume that there is a problem with the podcast.
The podcast plays fine in my desktop Safari on my MacBook.
Just so that you know, the preinstalled apps are only updated when the iOS is updated. There are never updates released for Safari, iTunes, the App Store and so on, except for the features that may be added when the iOS is updated.
I figured it out!!
When transfring your podcast from Garageband using the share with iTunes share option you can't select an audio setting higher than an Music Podcast. Higher qualities fail to load the plug in giving you the error.I don't know how that got changed in the settings but that was it.
Thank you, hope this helps others with same issue.