I've got 64GB of 200GB used, and it started having issues the other day. First it was intermittent, now I can't even get at my iCloud photo's. Tried on both my iPhone via safari/chrome browsers, dev and gaming workstations, and a couple of my laptops and all have the same issue.
Spoke with Apple Support and they said it's some sort of server side issue, but they aren't being very forthcoming with any information nor does it show anything helpful on the system status page here: https://www.apple.com/support/systemstatus/
They told me to wait several hours and try again...