WordPress Login takes me back to Login page

My main website, https://www.plumgrid.com has wordpress installed on it, and works perfectly. However, I created a copy of the website and placed it in a folder, and can view it at https://www.plumgrid.com/700devStaging007 The problem is, when I try to login to the Staging installation, the login page just redirects me to the login page again. I am unable to login to the wordpress admin I have a separate DB, separate User associated with it, and I have gone through 100 articles and solutions but still am stuck. Have deleted plugins, and renamed themes, and done everything possible.

Help please!