Are you also using Basic Authentication to protect your /wp-admin/ backend or just Post or Page password protection? Are you using the built-in WordPress method of password protecting a page using the option settings in the WP Post Editor or are you using some other method (another plugin or theme or builder option settings)? Regarding Basic Authentication – there is a known issue with Basic Authentication that is fixed by this htaccess code in the BPS root htaccess file: ErrorDocument 401 default
. That does not seem like the problem that is occurring though, but it is worth mentioning.
BPS has built-in troubleshooting methods. So activate the BPS plugin and do the BPS troubleshooting steps here > https://forum.ait-pro.com/forums/topic/read-me-first-free/#bps-free-general-troubleshooting and let me know if any of the BPS security features are causing this problem.