The situation
I still have an old 486SX25 system which I use to run multiple ssh sessions to different other systems. For instance to use IRC or view email. This way I can keep track of those things when doing things in fullscreen on my main desktop. The system started showing the following error on every virtual console:
eth0: mismatched read page pointers 4c vs ff
After a time I saw the connections to other systems fail.
The solution
When searching for the error message I found out that the error message is an indication for a broken NIC (Network Interface Card). Since it is an old system it still uses ISA (Industry Standard Architecture) slots and I still had some other cards available to replace it. But when I replaced the card, the kernelmodule couldn’t find the card on the given address (0x0280). To be able to fix this I had to boot the machine from a DOS boot floppydisk and insert the floppydisk that was part of the original card (PnP because it was ISA). This disk contained the ezcfg.exe on A:\CONFIG\EZCFG.EXE which could be used to change the address (and IRQ and duplex mode) the card uses. I used this to change the original (0x200) to the value I wanted (0x280). After this everything worked again.