Then had to sign in to run Mach3 as if Mach3 had just been installed.
This occurs when Mach3 changes the .xml file to an earlier backup (which can be caused by an incorrect shut-down, corrupting the running .xml).
If you are then using this corrupt .xml file (copying it to the new set-up) re-installing Mach and/or using different PC's will not resolve the problem.
IMO, the way forward is to search the Mach3 \ xmlbackups folder for an earlier .xml file (dated before the problems began) and try that (it needs to be renamed with the correct file extension etc.).
Failing that a fresh install (after deleting the Mach3 folder) and manually entering all the set-up details thereby creating a new .xml file.
Tweakie.