For reference this is when the change was made
e5eb98a

Raising the priority

in your custom /administrator/.htaccess please try adding:

ErrorDocument 401 default

And see if that helps in your server environment (to be clear, add this in YOUR CUSTOM /administrator/.htaccess and not in the Joomla .htaccess in /.htaccess)

Okay, that did help. But i think the same question will be asked a lot for all users that use Cpanel,DirectAdmin etcetera; the default password protect directories function in those systems doesn't add that line making the users backend unreachable if they wanna password protect it. Should Joomla add some docs about that to inform users?

This DEPENDS on your own server configuration.

Standard configurations of Apache will have a default handler for 401 errors - unlike your non standard configuration.

you might also need a 403 line too

ErrorDocument 403 default
ErrorDocument 401 default

For information Akeeba Admin Tools has this feature, and their forum and documentation talk about this issue and how to resolve it.

As Joomla doesn't provide any feature to secure your /administrator/ with a .htaccess and as this change to the code htaccess will work in standard apache configurations (ones where the webhost has not overruled the 401/403 handlers) I don't see that this is anything that Joomla needs to educate or document about.

Please can you tell me who your webhost is? (Dreamhost? Hostgator? Fasthosts? Bluehost?)

This also appears to be a common issue for DirectAdmin control panel users. Still - not a Joomla issue.

Suggest closing this.

Ok, thx closing it. Maybe i'll talk to the directadmin guys about it ;-) seems common on their default installations