Wp-admin infinite Redirect

Hello!

I’ve got a website which is running on a wordpress installation in the root directory of my webspace. Beside this I also have a second wordpress installation under a subdirectory in the root folder.

Here is an example of the content of my root directory:
/secondWordpressInstallation/
/wp-admin/
/wp-content/
/wp-includes/
.htaccess
etc ……..

When I try to login into my second WordPress Installation (which runs on a subdomain secondWordpress.domain.com , I’m always redirected to the same wp-login page where I was before!?

For example:
https://secondwordpress.domain.com/wp-login.php?redirect_to=http%3A%2F%2Fsecondwordpress.domain.com%2Fwp-admin%2Fpost-new.php&reauth=1

How can I fix this?

Thank you so much for you help.

Best regards