Stuart,
I have tried v5 myself as well but no luck.
I am fairly certain, the issue has to do with High Sierra and/or the new underlying filesystem.
I have contacted NCH support. After a few emails back and fort, they have sent me a development version of the application that has built in logging. I have run it and sent back the logs. This was nearly a month ago but I have not heard from them since, despite chasing them a couple of times.
I am using the free version so I am really low on their priority list. Are you using the free or the paid for version?