Error in Logs for Subscribe by Email

I'm seeing this error in the logs. We have been experiencing some database issues and I'm trying to triage them. I'm hoping you might be able to shed some light on what might be causing this issue:

[22-Jan-2016 16:34:55 UTC] WordPress database error Deadlock found when trying to get lock; try restarting transaction for query DELETE FROMwp_1795_optionsWHEREoption_name` = '_transient_sbe_pending_mails_sent' made by include('wp-load.php'), require_once('wp-config.php'), require_once('wp-settings.php'), do_action('init'), call_user_func_array, Incsub_Subscribe_By_Email->init_plugin, Incsub_Subscribe_By_Email->maybe_send_pending_emails, get_transient, delete_option

I'm on a WP Multisite, running 4.4.1. The plugin is activated on a site by site basis (not network enabled).