Minecraft launcher fails to load

The Launcher loads fine, but when I click "Play" it seems to be downloading 1.8.4.jar according to the status bar, then crashes.

The complete output from "Game Output (my minecraft name)" on the fourth tab is:

Error: Could not find or load main class net.minecraft.client.main.Main
Java HotSpot(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

Apologies for posting here if I've made an obvious error, but I have removed both Minecraft and Java, run registry cleaners and reinstalled from scratch several times without success, and spent 60 minutes searching for a solution without any success.

Can anyone help me?