I created a new user with the %Developer role and verified that the class could be debugged successfully. I then added the %SecureBreak role to that user and encountered an issue similar, but not identical to the one reported by @Rueben Van Mol and @Donna Goodall:

<COMMAND>zDebugStub+30^%Debugger.System.1
Invalid debugger target: ##class(Blue.App.Handler.Absolut).TestConnection()
Target has exited debugger