Intermittend login problems

Addition:

The website’s content is stored in directory \wordpress, and I have a file “index.php” in the root with the following content:
>>> …
>>> /** Loads the WordPress Environment and Template */
>>> require(‘./wordpress/wp-blog-header.php’);

I have got the impression now, that the problems with the login (to access a specific page) occur only when using index.php to access the website,

because with using .htaccess instead with the following content
>>> Redirect /index.htm https://[domain-name]/wordpress
all login tests so far have been successful.