Hi all,
thanks for your comments.

jdow: I am using lowest selectable bandwidth 550 KHz. I attach a complete log file (with parameters), see remarks further down. Maybe I can improve the settings with your help.

Siegfried: Ok, understood, thanks.

Jean: OK, fits somehow to Siegfried's comment. I will follow this up, thanks for the links.
A second power supply cannot be the a problem, as I have only one power supply connected on only one point to the Pluto. Pluto powers the OTG apdater and USB/LAN adapter, no redundant suppy with Y cable is used.

All: I have just had a freeze of the first kind (which could be repaired by stopping and starting the radio, no restart of the whole SDR Console needed). After stopping and starting the radio I was able to continue the QSO after a short break. 

I attach the log file. I think, the relevant log entry (corresponding to the freeze) was "21:26:08.880: Radio PlutoSDR> NOT Ready".