Since the ToC patch on Wrath Classic and with the HC test server I was getting fatal errors on every/any toon regardless of where they were located in game. on my desktop PC.
Classic era was fine so I know there was some sort of compatability requirement change in these new builds for wrath and the classic ptr.
My desktop is old:
Win 8.1
AMD FX-8310
970 mobo
R5 240 GPU
DirectX 11
All was patched up to current (if you want to call it that), but it was no use.
I was about to start pricing a new PC when I discovered I was still able to upgrade to Win10 for free, so I gave it a go and that solved it!
The only changes I can see are the OS upgrade and the inherent upgrade to DirectX 12.
So if anyone else is in this boat, this might be all you need.