The plugin is already on version 5.0.1 (at this point), a there is still problem with it. (SEND button has just stopped working)
Rolling back to a earlier version worked for me.(v4.7)
Unfortunately, even if using a “Disable All Updates” plugin, the Contact Form plugin continues to update itself.
So I’ve installed “Easy Updates Manager”, and disabled specifically Contact Form’s updates.
Right now I am using version 4.7, which keeps my forms running without further problems.