The issue was caused by mod_evasive. There are two workarounds for this issue. The first is to add a DOSWhitelist entry for your IP address in /etc/apache2/conf.d/300-mod_evasive.conf. The second is to uninstall ea-apache24-mod_evasive. I recommend the first workaround. I temporarily whitelisted my IP address and confirmed that the plugins were updated.