Seemed to kill a self-hosted site running WP 3.9.

Where are you hosting, and what other plugins are you running? I see that you are having issues with the All In One WP Security & Firewall plugin as well, have you tried with Login LockDown on it’s own?

-Michael

Login Lockdown was the first plug-in installed.

After removing it I switched to All In One WP Security & Firewall which works GREAT with exception of minor difficulty I have with file-permissions function. So far I put All In One WP Security & Firewall at about 98%-99% of full functioning.

I also have added Jetpack and Google Analytics.

I’m sure all of those were coming after Login Lockdown.

Incidentally I see very short-lived error 500’s very occasionally with various WordPress functions and in those cases a page-reload resolves the issue (which makes me wonder if there is a performance/response time causality to those).

In case of Login Lockdown any access to the login pages was flat-out dead in its tracks: reloading/retrying pages was of no help.

BTW I’m also trying Contact Form 7. That has been the very latest effort and long-time after trying Login Lockdown.
I’m using a third-party theme from a design house that sells themes, although it didn’t appear to be something that you asked about.
Out of possibly excessive sensitivity to security I’ve been reluctant to mention the commercial hosting company I use.

Note: that error was when attempting to display the login page

This is the default wp-login.php, correct? Not something custom within the theme?

I’m using a third-party theme from a design house that sells themes, although it didn’t appear to be something that you asked about.

Have you tried switching to one of the default themes, just to see if the login page works afterwards? And are there any errors being generated in the error logs that might point to what is going on?

-Michael