The plugin was working as expected until I updated WP. Since updating it stopped working. When you enter the password the page would reload and still display the enter password content rather than the protected content. I thought it may be a conflict with a plugin or theme so I tested it on a clean install with no plugins active on the WP 2016 theme and it worked as expected. I then activated and deactivated one plugin at a time until I found the culprit. Once I activated WP Super Cache it stopped working and I got the same issue.
To create a work around I entered the page URL into WP Super Cache advanced settings to not cache that string. However, I am pretty sure there is a simple coding change that would fix this issue. If I had time I would debug it myself, however, I am strapped for time at the moment.
If you could let the developer know so that he can include a fix for this in the next release I would appreciate it.
Have a great day!