Password protected page not accessible / wp-login.php?action=postpass error

Hello there,

the wp-login.php?action=postpass error has been discussed in this forum but as far as I could see never solved.

A password protected page, e.g. https://mydomain.de/test/
password 123

Configuration:
Wordpress 4.9.6
just added SSL
no caching plugins

if password is entered, a blank page appears with the following URL: …domain…/wp-login.php?action=postpass

affected are all browsers (Firefox, Safari, Chrome on macOS)

hitting the “back” button in the browser and a subsequent reload of the page makes the content of the protected page visible, so the password worked and right to access is saved

the following actions do NOT change anything (all tried with browser cache deleted every time):

• turned off/on Force SSL in wp-config
• turned off/on WP all-in-one Firewall & Security plugin
• turned off/on permanent https redirect in htaccess
• re-install WP 4.9.6 via backend update feature
• update permalinks
• suggestion from other forum post to overwrite wp-login.php with a file from a working WP installation

I tried “passster” plugin to partially protect content, but this not working either (returns “false password” error although password is correct).

I really could use some help here, thank you for any ideas!!!