Uninstall current java versions, uninstall runemate and runescape legacy and then try to install (when you install runemate it download the correct java version), try to create a new windows user and try to run it there, be sure that runemate and osrs files are in the same disk C: in my case, desactivate firewall antivirus, try to run it as admin. In my case it solved very randomly after i tried all those options for 2 days, i uninstalled one drive then download runemate (to let runemate install the correct version of java) install legacy mode and it worked, like i said i was trying all those option for 2 days and one time it worked so good luck man. When you uninstall java make sure to uninstall all the versions and when uninstall legacy mode search for more files of runescape and delete them.