Error: E_AUTH_USER_ALREADY_REGISTERED

I have some Acsm files bought from Sony to use on my Sony reader PRS-350 When I try to download these to my reader, or if I click onto the file to read the book on my computer I get

Error: E_AUTH_USER_ALREADY_REGISTERED. Please can you help me sort out this problem. A previous reply was to select edit preferences on Sony ereader to de-authorize however the dropdown menue I get does not have a de-authorize button