Discussed here: Version number corrupted after upgrade? - #28 by dms
The v65 tag wasn’t annotated properly in the repository and has been fixed.
You can fix this by fetching tags and then updating as normal:
cd /root/mailinabox
sudo git fetch -t
curl -s https://mailinabox.email/setup.sh | sudo bash
Both steps might not even be necessary. The fetch gets the tag to your system and that might be enough to silence the error. The normal update might also get the tag, but I figured running an update would be a good idea to make sure everything is in place appropriately.