@bido Well where its failing we can rule out a lot of things. You have your PXE booting configured correctly because iPXE is running. That is undionly.kpxe. That file is getting transferred to the target kernel and is running. Its iPXE that is having a hard time.
The next test I want you to do is to install a cheap unmanaged switch between the target computer and the building switch. This test will give us an indication if spanning tree is enabled, but you are not using one of the fast spanning tree protocols (RSTP, MSTP, port-fast, FastSTP or whatever your switch mfg calls it). The dumb switch will keep the building switch from resetting the spanning tree counter while booting. Understand this is only a test to tell us if its spanning tree or not. My gut reaction is yes.