I am having a 500 internal error on my site - http://www.citywomenflaward.co.uk
I installed W3 Total Cache and updated Jetpack on http://www.citywomenflaward.co.uk yesterday at the same time your Defender (and other plugins) were installed.
I did all the usual to fix – disabled all Plugins and enabled one by one. Ran a WP_Repair on the config file.
Deactivating Jetpack stopped the 500 – but I need the Jetpack because I use it to pull in posts from one citywomen.co.uk to citywomeflaward.co.uk and vice versa.
Activating Jetpack but de-activating Defender and de-activating W3 Total Cache stops the 500 error message. (activating the WPMYU Dashboard does not throw a 500).