Installer is broken - "Please fill in all the blanks"

The installer doesn’t work. When entering database credentials it says „Please fill in all the blanks.“ even though I filled in all required fields.

I don’t think it’s a problem with sessions not being writable as other posts suggested. This is a server with other applications running on it that works fine in that regard. And even if it was the error message is wrong.

Also, the checkbox „Don’t verify server certificate“ becomes unchecked after submitting the form. Could the problem be that the installer doesn’t respect that setting and tries to find SSL certificates even though I check the box to not verify certificates (since the server is on localhost)?