Just booted with "intel_idle.max_cstate=1" added to the kernel cmdline. This seems to have fixed the problem. Network speeds are as expected, ethtool shows no errors after some big network transfers. Tested the kernel from #59 and the default 5.8.0-55-generic, both work when "intel_idle.max_cstate=1" is set.
Just booted with "intel_ idle.max_ cstate= 1" added to the kernel cmdline. This seems to have fixed the problem. Network speeds are as expected, ethtool shows no errors after some big network transfers. Tested the kernel from #59 and the default 5.8.0-55-generic, both work when "intel_ idle.max_ cstate= 1" is set.