Some more comments to this great post of acesuares:

1) the log file of the RRD error message is /opt/zimbra/log/zmlogswatch.out. In my case it was sufficient to delete one file which was not updated since the day of the migration. After this (and zimbra restart) Server Status and Statistics did show up in the web interface with no error message

2) the error message about the "Detected duplicate databases" is in the file /opt/zimbra/log/clamd.log
For me it was also sufficient to delete the cld file