multiple sign-in switched on.
You also have a Google Apps account firstname.lastname@example.org.
You are logged in to your Google account, email@example.com. This account does not have access to any sites in appsdomain.co.uk
You receive a link to look at an item in a site in the Google apps domain appsdomain.co.uk.
You click on that link.
You see >
so you click on the 'sign in as a different user' link (https://www.google.com/accounts/AddSession?service=jotspot&continue=https://sites.google.com/a/ki-network.org/memberspace/home) and you get
Which is not very helpful, it gives you no indication of what you have done wrong and/or how to proceed.
(I believe the Google engineers are working on improving this).
What you need to do is log out of the account firstname.lastname@example.org and log back in as email@example.com
or switch on multiple sign-in - but make sure you understand the ramifications before you do, read the warning !