Apple Hardware Test boot error -1003D

After resolving a memory issue on EMC2428 iMac, I want to boot up AHT to make sure nothing else is wrong with the iMac, however I can't seem to get the tool to boot on this mac. Upon trying to boot AHT using Opt-D, I get the globe and then an error "-1003D" on it.

Other things I tried: "D" at startups does nothing, tried copying AHT files in .diagnostics folder (https://github.com/upekkha/AppleHardwareTest) -- still nothing.

Any idea what that error is or why I can't boot local diagnostics tools either?