Hi Gus,
it wasn’t invalid per say, but it’s last bit of the first byte was set to 1, which means a Multicast Group Mac address which I think causes the router not to assign it an IP address, anyway I have uploaded a snippet to Codeshare that will generate proper Mac addresses based on Unique String, the code will generate a unique address per board 
here it is :
https://www.ghielectronics.com/community/codeshare/entry/822
feedback would be great…
Cheers,
Jay.