Flickr cookies

[Astraeus]

I sent out some invitations to several family members to view photos of a family outing on Flickr and several signed up successfully but my dad has a problem whereby he has signed up but, once he goes and logs into Flickr, a screen comes in which states:

"Your registration was successful"

Only to go on mentioning removing Flickr cookies or enabling cookies and prohibiting him from gaining access to the photos.

Anyone know what we can do about this?