Same problem started today. Yesterday it was fine and only thing that changed on my PC is windows 10 updates - could they relate to this problem?
Ok i fixed this problem atleast for myself. The problem probably was that my friend installed jdk 11 yesterday and added an 'Environment Variables" PATH to C:\Program Files\Java\jdk-11.0.2\bin; and i removed this part and restarted RuneMate and boom it worked. So the problem was probably it was trying to load different version of java or something and didn't work because of RuneMate comes with its own java shipped in.
To access 'Environment Variables' in Windows 10 goto Control panel -> System -> Advanced system settings -> Environment Variables -> And check if your PATH has something else than %USERPROFILE%\AppData\Local\Microsoft\WindowsApps and remove the "excess" ones and see if RuneMate starts working.