Nucleo board can't program or debug after upgrade.
Since it is not possible to uninstall openocd or to select for use a previous installed version from System Workbench, I had to copy the previous version’s binaries from the folder:
C:\Program Files\Ac6\SystemWorkbench\plugins\fr.ac6.mcu.externaltools.openocd.win32_1.17.0.201801120948\tools\openocd\bin
and overwrite the latest, not working binaries in:
C:\Program Files\Ac6\SystemWorkbench\plugins\fr.ac6.mcu.externaltools.openocd.win32_1.18.0.201807130628\tools\openocd\bin
Now everything works again.