It can be a temporary server problem. If that's the case, you only have to wait for the Yahoo! team to correct the problem.
At the moment I write this, I have this message:
"Sory, some functions are not available (temporary problem). Try again later."
Try to export your contact list to a file on your computer.
Open your contacts list, chose "Tools"->"Export..."
Chose one of the format available and take a note of exactly what format you used.
You may want to export in more than one format.
Once that's done, check that all informations are good and make any required correction.
Import the contact list back into Yahoo! Be sure that you import the same format as the one you used to export.
Électro, August 2010
