Nokia :: N900 - Skype IM - Unable To Deliver Message Contact Unauthorized

I'm having a problem with Skype on my N900 (PR1.2).

The vast majority of my Skype contacts are fine. I can Skype Instant Message them with no problem.

However, there are a couple where when I try and send an IM, I get the message:

messaging_fi_unable_to_deliver_message_contact_unauthorized

I have tried removing the contact (from both my and the remote contact's Skype), and I've readded from my N900 and asked for authorization, and its been accepted, but I still get the error message when I try and IM.

When the remote contact IMs me the message never turns up, but they don't get an error message.

I've also tried it the other way around, where we delete the contacts on each other's Skype, and then the remote contact has asked for authorization, and I've accepted on the N900 - but still the same message.

The remote user us on Windows 7, using Skype 4.2.0.169. It has worked previously, but all of a sudden, it stopped for no obvious reason. This is happening with two of my contacts, who are in two different locations on different versions of Windows. Oddly, Skype voice calls work okay!

I can IM all my other Skype contacts no problem.