Post by Ken Sandvosscannot open address book, error parameter is invalid
Look in the Eudora data folder, and with Eudora closed, rename the file NNdbase.toc to NNdbase.toc.old.
Try again. If the Address Book now opens, that Address Book file was corrupted somehow, and it should now be OK.
If it still doesn't work, also rename NNdbase.nnt to NNdbase.nnt.old.
If that works, that file is corrupted too, and you will need to restore both files from a backup, if you have one.
HTH. Cheers, Dave.