I realise its an old thread, but I appreciate that you looked at it
Our problem occurs, according to their feedback, that when they try to comment on someones picture, the get the error. I havent been able to replicate it to a useful extent, so cannot really test it.
I have my suspicions that the connections the people getting the error use have some influence but it doesnt make sense along with what you said. Can any form of cookie problem cause that there is not photo var in the link?