Problem:

This is common error reported on Windows Server, due to invalid format of Document Path in the oneadmin/config.php.

Solution:

Replace single backslashes ' \ ' with double backslashes ' \\ ' in the Document Path. For example, the correct format is:

$path["docroot"]="E:\\home\\sitename\\httpd\\oneadmin\\";

Save the changes, and refresh the Login page, it should be fine then.