Thank you for the really fast fix...
I downloaded both "brickd_1.0.3.dmg" and "brickd_latest.dmg". Installation completed successfully, but brickd was still not running, with the same error message in /var/log/brickd.log ("The process has forked and you cannot....")
Then, I downloaded the source and was able to start brickd with
sudo python brickd_macosx.py
After this, Brick Viewer could connect to brickd.
Might be a problem in the installation package? Or do I need to uninstall before installing the newer version of brickd?