I am trying out the free version of this plugin on a news site where one category has payable content, using WordPress 3.9.1. and Membership Ver 18.104.22.168. The following bug is however noted (using FireFox Ver 30.0) on the Admin side at Options > Content Protection:
Editing of the content for "Protected content message" and "No access message" keeps disappearing when switching between "Visual" and "Text" modes.
At "Protected content message" the entire content box disappears while the editing tools at the top remains fixed in "Text" mode after selecting "Visual". Refreshing the page, returns the content in "Text" mode, however when switching back to "Visual", the content box disappears with the editing tools staying in "Text" mode.
At the "No access message", it is almost the same, but at least the content box remains intact (the box doesn't disappear) although the editing tools remains fixed in "Text" mode.
It seems like the code used for "message editing", used by various plugins, is incompatible with WordPress 3.9.x. This is noted with several of the older plugins where quick-fix attempts were made to update the plugin to be compatible with the latest version of WordPress. Although the plugins itself works perfectly fine with the latest version of WordPress, there is an issue with the older code used for messages editing from the admin panel of the plugin.