I've installed Joomla and started configuring the site.
After changing the Administrator user's name (not login name!) I could no longer login into the site's backend (domain.name/administrator).

Entering correct credentials just lead to the login page again.
However, Joomla reacted to incorrect credentials - in this case, a message was displayed: "Username and password do not match or you do not have an account yet."

I've searched the internet for this problem and found many different reasons, double-checked all suggestions and nothing helped. In particular, the required modules were enabled, tried disabling .htaccess, etc.

In a HTTP debugger I've noticed that there were multiple session cookie names at the same time.

After some digging, I've found the file libraries/src/Session/Session.php and line 920:

		$this->_handler->setName(md5($options['name']));

The debugger showed that $options['name'] is already an md5-hash calculated elsewhere (CMSApplication.php, line 738: 'name' => \JApplicationHelper::getHash($this->get('session_name', get_class($this)))).

I removed md5 and everything works fine now, i.e. changed libraries/src/Session/Session.php, line 920 to:

		$this->_handler->setName($options['name']);