Thanks - gave that a shot, but unfortunately nothing new in error.log. Also doublechecked nginx and php-fpm, nothing useful there.
Is it worth trying to hack some extra verbosity into AuthLoginController.php? My php is very rusty, but I can give it a shot.
2022-01-06 17:19:58 Error: [Cake\Http\Exception\InternalErrorException] The authentication failed. in /usr/local/www/passbolt/src/Controller/Auth/AuthLoginController.php on line 93
Request URL: /auth/login.json?api-version=v2
Client IP: 172.22.0.5
2022-01-06 17:19:59 Error: [Authentication\Authenticator\UnauthenticatedException] Authentication is required to continue in /usr/local/www/passbolt/vendor/cakephp/authentication/src/Controller/Component/AuthenticationComponent.php on line 177
Request URL: /auth/is-authenticated.json
Client IP: 172.22.0.5
2022-01-06 17:20:00 Error: [Authentication\Authenticator\UnauthenticatedException] Authentication is required to continue in /usr/local/www/passbolt/vendor/cakephp/authentication/src/Controller/Component/AuthenticationComponent.php on line 177
Request URL: /auth/is-authenticated.json
Client IP: 172.22.0.5
2022-01-06 17:20:01 Error: [Cake\Http\Exception\InternalErrorException] The authentication failed. in /usr/local/www/passbolt/src/Controller/Auth/AuthLoginController.php on line 93
Request URL: /auth/verify.json?api-version=v2
Client IP: 172.22.0.5