In the meantime I have found that the "SPI_Dev_Init" related errors came from not having enabled SPI in /boot/config.txt. After uncommenting the line #dtparam=spi=on those error messages disappear.
The first error message can be avoided by adding a line to the procedure Get_CPU_INFO_Init:
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$202082 : cpu_rev:=RPI_SetInfo(cpu_rev,'3B',4,1,2,47,40,1024); // Pi3B (a02082 Sony, UK)
$2020d3 : cpu_rev:=RPI_SetInfo(cpu_rev,'3B+',4,1,2,47,40,1024); // Pi3B+ (a020d3 Sony, UK) <---new line
else LOG_Writeln(LOG_ERROR,'Get_CPU_INFO_Init: (0x'+Hex(lw,8)+') unknown rev:'+cpu_rev+': RPI not supported');
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It is not yet clear to me, whether more has to be changed.
Regards Kai