Fejes József [off-list ref] :
[...]

The paste is from 3.11.6 actually, but 3.12.6 produces the same
behavior. Nothing changed recently in my networking setup.
Immediately, when I plug in my laptop to that interface, the error is
produced, ifconfig on the machine shows that packets are sent out, but
I see no received packets in my laptop on the other end.

Ok.

Could you check the irq count and ethtool -S eth0 as well ?

Does the laptop also include a different operating system ?

I haven't used that interface for quite some time, a year at most. I
keep up to date with the stable kernels, so a version change broke it,
but I'm not sure which. Would it be the best course of action if I
tried some older versions?

Yes, please do.

There has been no specific change in this timeframe for the chipset you
use (RTL_GIGA_MAC_VER_25). The problem may have been unnoticed for a
rather long time. :o/

-- 
Ueimor