Why I get this error 0xd126001 (http://ipxe.org/3d12001) using WDS?

[robinsmidsrod]

Based on the error code you copied it seems like your TFTP server is not responding properly to the URL you specified. It is known that the WDS TFTP server is a bit picky about using forward-slashes in paths, so you might need to use escaped backslashes (%5C). The URL should then look something like this: tftp://10.10.10.1/%5Creminst%5CBootx86%5CImages%5Cboot.wim

Although, if you're loading boot.wim directly I'm assuming you're using wimboot to load it. If that is the case, I'd load all of the files mentioned in http://ipxe.org/howto/winpe or http://ipxe.org/howto/sccm using HTTP URLs instead of TFTP. Microsoft IIS is much more standards-compliant than the WDS TFTP server.