The ProQuest Digital Dissertations database has moved to the ProQuest platform, and is no longer available through the URL you entered.
If your library subscribes to the dissertation database - now called the ProQuest Dissertations & Theses (PQDT) database - you should be able to access it from this URL:
If your library subscribes to other ProQuest databases, you will find PQDT listed as "Interdisciplinary - Dissertations & Theses" in the Database pulldown menu. If you have difficulty accessing the database from the above URL, please contact your library reference staff for help accessing the ProQuest Platform and PQDT in particular.
If you are not affiliated with a library that subscribes to PQDT, you can search for dissertations and theses through Dissertation Express. Dissertation Express lets you purchase unbound print copies or PDFs of graduate works published by ProQuest.