I've just updated the Membership 2 Pro plugin and the site is now unaccessible. After using the cPanel file manager to rename the plugin folder I can get back in (it deactivates the plugin) but clearly I need it back! (This is not a public site so I did not take a backup as I normally would (I will next time!) - I have renamed it to hide the site as it contains sensitive information, hence my use of the plugin)
It is adding /?ms_flushed=yes after every URL (which is why I figured it was this plugin) - is that a clue?