If it is ok to remove previous messages between users, easiest way is to check Front End PM PRO > Settings > General > Delete data on uninstall, then uninstall and delete this plugin from admin dashboard. Then install again.

This plugin creates 4 db tables and save some options in option table. All are prefix with fep_. You can delete those from db and it will automatically start again from the beginning. If you follow above steps then that will take care of deleting all of these.

Let me know.