Thank you for your support with this issue. I tried a restart, but that didn't fix it. So I tried disabling all of the anti-virus "shields", powered the PC down (completely) and then back on again. At this point, Engen started to work again - and, interestingly, continues to work even after re-enabling the anti-virus shields!
I still have no idea what is going on but, as JimH suspected, it does seem to have been affected by the recent anti-virus update.