Disabling trailer protocol when using this W5100-based Ethernet shield
Are you sure that the trailer isn't just there to pad the packet to the appropriate size, or as some form of CRC checksum?

In fact, looking at the RFC it looks to me like a host, which wishes to use trailers, would send the trailer request during ARP, and if the other host understands trailers then it will add it to its list of hosts which understand and use trailers. Therefore in theory it wouldn't be the trailer which was causing your problem. However, I can't categorically rule it out as this is based on my memories of my networking course and a superficial reading of the RFC you linked to and Wikipedia.