icloud photos application won't open

My icloud storage is almost full, so I am trying to download my photos to my iMac (which is too old to automatically connect to icloud). When I go to the icloud web application it will not open Photos. I first get a message saying, "Network unavailable or slow. Photos is taking longer to load than expected." Then the spinning wheel of doom stops and I get a message saying, "Can't load Photos. There was a problem loading the application." I'd really like to download my pictures! Any ideas?