I found the reason but don't know how to fix it. In system preferences, security at "Automation" there should be an entry for Path Finder that allows to control Mail. On my MacBook it is there, on my MacPro not.
Based on what I've read, but unfortunately don't fully understand, there are new APIs to handle automation permissions. You can reset all Apple Events and start from scratch, but I wouldn't want to do that. I suppose I could try to re-install Path Finder to see if that would prompt something. Regardless, I'd really like to see some progress or even a response on this and other issues that have been sitting for months now.
@nti how cool that you found the possible culprit - but you are right, one can't anything there easily. Maybe we need a special version of PF that makes sure that feature is set. I did uninstall and install - but no change