I believe it must be a conflict with one of your plugins, as it is working for others.
Have you installed or upgraded any of your plugins lately ?
I would suspect it could be gravity forms as this plugin offer similar functionality, but I am far from sure.
you could try and deactivate the last plugins you installed/upgraded to see if it helps
alternatively uninstall Frontier post, and reinstall it from the plugin directory again.
subscribers and contributors to upload media
I did that in the first versions my changing capabilities on the subscriber role, but since that will impact all of wordpress I removed it again.
If I am adding one or more Frontier roles to the plugin, do you need more than one - In other words do you both need a subscriber and author type custom role ? and if so can you tell me why (trying to understand how people are using it)