Are you able to add or delete any users at all from the list?
If you /ignore a nickname, or add or delete one through the Address Book/Control dialog, exit mIRC, and then run mIRC again, has the list changed? If it has not, the most likely reason is that mIRC does not have access rights to the file.
You could delete the file and try running mIRC again to see if it can create the file and write to it.