Home sharing could not be activated (11556)

Anybody any ideas what this is about?

I have inherited an iMac from my parents (early2009). I am trying to get it to access my main computer's library (MPB, both running Lion, both iTunes 10..5.1 (42)).

I have full admin rights on the iMac.

In iTunes, sharing is enabled and sharing is shown in the sidebar ('Home Sharing'). Clicking on Home sharing it asks me for my apple ID ... then tells me that it could not be activated because an error occured (11556).

I have done a search but most of the hits relate to iTunes 8 (a couple of years ago). They generally talk about de-authorisation ... I have no idea how to do this ... I do not know what ID was used when it was authorised ... and cannot find a way of hosing iTunes completely.

Another observation probably not related ... the preferences general tabs are different.

My MBP, bought from Apple, talks about 'Films' and 'TV programmes' while my parent's iMac (bought at their local UK store) talks about 'Movies' and 'TV Shows'. I do not think that this is related since the two have not begun to talk to each other yet! Is there a localization setting somewhere?