Looks like it is conflicting with something. Please check your server log to see if there is any errors being generated (the details from that log file will be very useful).
Also, do a plugin conflict test:
1) Temporarily deactivate ALL the plugins except the simple membership plugin.
2) Temporarily deactivate ALL addons/extensions also (if you are using any).
3) Test to see if the plugin behaves how it is suppose to.