Steve Langasek pointed me to #294867. It seems that de4x5 module is somewhat deprecated and considered broken most often.
Joey mentioned in the same bug he's considering dropping de4x5 for d-i. I'm not sure that it would be safe to do it on all occurrences of this module in discover1-data, but at least for your specific hardware, it would be interesting to test that. So, can you please try "modprobe tulip" on console 2 if the loading of de4x5 still fails ? And, anyway, getting the output of "modprobe de4x5" would be interesting. Thanks for your contribution. --