momomo01

yes.
I tried a couple of times, creating new sites, but every new site will not work.

the other site “https://fingerhut.me/wordpress/testsite” does not have a domain name mapped to it.

I’m not sure if I understand you correct.
this is my setup:

I have my main domain https://www.fingerhut.me

when I login as super admin, under “My Sites – Settings – Domains” I have added the domain “ohfuton.com”.

under “My Sites – Sites” I have a site with the path “fingerhut.me/wordpress/ohfuton/”
Here the Domain is “https://fingerhut.me” and the path is “/wordpress/ohfuton/”
” Update siteurl and home as well.” is activated

In the admin panel for “ohfuton.com” under “Tools – Domain Mapping” I have also added “ohfuton.com” and set it as primary domain

Thank you Mika for investing your time and efforts!
my .htaccess is as follows:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ – [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ – [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>
# END WordPress

I temporarily deactivated mu-plugin (I’m using this modified version https://patchlog.com/wordpress/wpmu-domain-mapping-in-subfolder-install/ )
and also temporarily changed the theme of the main site to “twenty fourteen”.. both did not have any impact on the behaviour of the login page.

hi, there is no error. the login page just loads again, with my user name written in it, but no password.
i noticed that the link in the address bar changed to this https://fingerhut.me/wordpress/wp-login.php?redirect_to=http%3A%2F%2Ffingerhut.me%2F%2Fwordpress%2Ftestsite%2Fwp-admin%2F&reauth=1

thank you for your reply.
yes. but i cannot login under this link, even with super admin.
in fact, i cannot access the dashboard for my test site anymore.
when i’m in the admin panel of super admin, I click on “my sites” then “Test”, it will lead me to the login-page, but I cannot sign in there.

The Template is “twenty fourteen” and the content should still be the standard “Hello World”.