Debugging code doesn't work

Hi Guys

Two of the HC servers I manage (a mirrored pair) have lost the ability to debug code, and I am unsure as to why this has happened. Clearly something has changed, as the other three are working just fine.

When I select a Class Method as debug target, add a few break points and run it, I get the following Output

Debugger executing '##class(InPhase.Test.CanUpdate).TestBIU()'
Executing ##class(InPhase.Test.CanUpdate).TestBIU()

<COMMAND>zDebugStub+40^%Debugger.System.1

Program has terminated.
Target has exited debugger

The classmethod being Debugged  ##class(InPhase.Test.CanUpdate).TestBIU() outputs a via lines via the WRITE command, and when I run it from Terminal it clearly works.

Can anyone help?

Andy