Advanced Settings - but note that this field (Remote CDR DB Password) is hidden.
Look for DisplayReadONLY and ChangeReadOnly near the top of advanced settings -- turn these on
Then change Remote CDR DB Password to your original password (I found my original pw in /etc/asterisk/cdr_mysql.conf)
Then apply ==> joy!
Note that the backup/restore had applied the mysql pw from my original server - but had not replaced the mysql database - so I had to put the pw from my new server into advanced settings to get going again.
this worked for me after I had restored my original asterisk server's full backup (backup had everything less CDR database) to my new server. - initially after the restore I was unable to apply config - yielding the error which started this thread. Specifically Access denied for user 'freepbxuser'@' while trying to apply config after restoring from one server to another one.