Hello jdow and Jean,
thanks for your hints. 

jdow, I have not noticed any reboot activiies by the Pluto yet, but maybe missed it.

Jean, I have saved only one log file up to now. At the end of this log file I found "Radio PlutoSDR> NOT Ready" as third-last entry, but no "Radio PlutoSDR> Ready". At restart of SDR Console the Pluto answered as usual, but I do not know for sure, if the Pluto had or had not rebooted in the meantime. 
At least in the "case 1" freezes I assume no reboot of the Pluto, because the setup worked again very quickly.
I will do further investigations, following your hint to enable the System Debugger Diagnosis. I will report my results here.

Any further comments are welcome.
tnx es 73 de Nils DHØHAN