Per Oren:
Try again. Something broke.

Google migrated my Photos/Albums from Picasa to
Google-Something-Or-Other - but they failed to migrate permissions and
are throwing a 404 if somebody does not have permission to view an
Album.

I set Permission=Public and am hoping it will work.

"Hoping" because I don't have another UserID to test with...

Somebody say something if I got it wrong.
--
Pete Cresswell