After doing some extensive research and testing, I finally found the cause. Apparently there was a custom MODSEC rule that caused this issue. I whitelisted the rule, then it began working fine again.
APACHE/LOGS/ERROR_LOG
[Tue Jul 02 16:14:33.651483 2019] [:error] [pid 5233:tid 47730625201920] [client 24.192.180.182:52199] [client 24.192.180.182] ModSecurity: Access denied with code 402 (phase 2). Operator EQ matched 0 at ARGS. [file “/etc/apache2/conf.d/modsec2/lw_virtual_patches.conf”] [line “41”] [id “5000214”] [msg “wordpress login missing action and or wp-submit arguments.”] [severity “WARNING”] [tag “critical_wordpress_protection”]