Access Denied - Sharing Folders

Hi

I am trying to share a folder on our sharepoint site with an external users. I provide the link and they are asked to confirm their email and enter the code. This is successful, the folder with contents briefly appears on the screen then it disappears and they get "Access Denied" xxxx does not have permissions to access this resource although they are being given access to the folder and its contents? Has anyone got any ideas?

Thanks

Alistair