Thanks for the hint with a new version of TD-15 I haven't read before.
Unfortunately the 550000 didn't ring the bell but that's what stands
in /etc/syctl.conf configuring the kernel buffer at startup so far.
The relevant text in TD-15 is:
Defines the kernel receive buffer size for this multicast channel in bytes. The default
value is 4000000. Increasing the buffer helps avoiding losses if the client is
temporarily blocked in IO operation and cannot read the incoming multicast fast
enough, e.g. in case of heavy disk load. On Linux system the following system
parameters in /etc/syctl.conf must be set to the same or higher values:
net.core.rmem_max=8000000 (reboot the system after a change)
As I just copied the new client and adapted my cast-channels.ini files
the 550000 limit in /etc/sysctl.conf was left untouched. For the moment
there is only a *.rpm but no *.deb of the new client distributed for an
automatic installation under Debian (or Devuan that I use now). So I
adapted /etc/sysctl.conf setting read and wtite buffers to 8000000 and
rebooted/restarted with receive_buffer_size=8000000 for every channel.
Now the system accepts receive_buffer_size=8000000 without complaining.
So I can make now a more meaningfull test of TC 2.14.4 versus TC 2.24.1.