@fogbrain That message has 2 causes.

It can’t get an IP address It can’t make a http call to the fog server.

If you watch it boot you should see it get an IP address, if yes then that rules out #1. The second is the FOS Engine will attempt to reach the fog server (as you posted in the error above). If it can’t it will start the sequence over again of querying for dhcp then attempting to contact the fog server. It should do this 3 times then give up. You need to test from the same subnet as the pxe booting computer if you can reach the url you defined in your original post with a different computer on the same subnet as the pxe booting computer.

I don’t think its a spanning tree issue right now. That would usually (but not always) show up in the FOG iPXE menu not loading. You do need to have your switch ports in portfast mode. You can test to see if its spanning tree issue by placing a dumb (read cheap, unmanaged) switch between the building switch and the pxe booting computer. That cheap switch typically would not have spanning tree plus it keeps the building switch port from winking as the pxe booting computer starts up.