The built in Windows Defender won't block MC enough to cause that. Something else is going on. It certainly seems like your MC installation is corrupted. It won't run right if JRTools can't load.
Uninstall MC. If you have both an old version of MC18 (or earlier) and MC19 installed, you may need to run MC19 first to make this work. If that doesn't help, reinstall using the latest MC19 build, and then uninstall.
Reboot. Don't skip this step because you are lazy.
Reinstall MC using the latest build from this board.