admin-ajax.php returns 500 error – unable to load nag

Wordpress 5.1

As of a few weeks ago, article countdown nag mysteriously stopped working on my site. For ALL posts, we are receiving an error similar to the one listed below:

Failed to load resource: the server responded with a status of 500 (Internal Server Error) https://www.allotsego.com/wp-admin/admin-ajax.php?action=maybe_display_countdown&post_id=101090

There have been no theme updates. I have attempted to delete article countdown nag entirely from the server, reinstall it, and still no progress. I have disabled every plugin with exception to leaky paywall and the countdown nag and am still getting the same errors. I have also tried changing the directory and permissions of admin-ajax.php in the class.php document and it is still returning a 500 error. I have also reverted Leaky paywall back 2 various iterations and it still failed to work.

I am at a loss at this point as to why we are getting this error and the countdown nag is failing to display. Any guidance would be greatly appreciated. Thanks.

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