@nilshg , thanks for helping.
I have more than one environment, and I can indeed update Pluto to version 0.19.4 in a specific one. However, on one computer, I do not use a large number of packages, so I keep using the default environment as well. I guess this is not good practice, but before yesterday’s release of version 0.19.4, I had no problems updating my system (which has occurred for more than one year now).