I was having the same issue, until I completely uninstalled all forms of JRE / JDK (Java). It seems you might be running a newer version of Java like I was. Try completely clearing Java off your computer;
-Uninstall all Java version
-Uninstall all RuneScape clients
-Uninstall RuneMate
-Clear Jagex Cache
-Restart Computer
-Reinstall RuneScape Client
-Reinstall RuneMate (this automatically downloads and installs the client compatible Java version)