quoted

I'm willing to do some debugging but unfortunately I'm not anywhere familiar with this driver and network controllers in general, therefore I'm asking for some hints/advice first.

This driver hasn't seen functional changes for ages. Any previous kernel
version that works fine so that you could bisect? It will be hard to
find any developer who has test hw, especially as your issue seems to be
system-dependent.
Please provide a full dmesg log, maybe it provides a hint.

I have a few active 80486 test systems (the 5.12.7 kernel works fine
on 80486) that I might be able to help test with too.

- Matthew