I found out meanwhile:
The F5OEO FW stores information somewhere in the flash that is not overwritten by an FW update.
I've a second pluto and FW beta 2020-02-05 works. The other -SDR-Console is not working pluto- was programmed again with
1. analog devices original FW 0.31
2. F5OEO firmware 20191220 (and it worked!!!)
3. F5OEO firmware beta 20200205, and fails again to operate with SDR console.
I checked the pluto.php site on my browser and found out that my individual settings (callsign) I did store
to flash using the F5OEO firmware are still available. They survived all programming I did!
So I assume that something is stored somewhere in the file system that causes the problems and crashes SDR-Console.
Does someone know to reset the pluto to "factory defaults"? My search was not successful up to now.