Twickland, Lennart, David – thanks for your replies - see below
Twickland - that's good to know
Lennart - there are no restrictions in the privacy pane of the Client preferences (on the client)
David - I'll try to ignore the aggression in your first paragraph and give you a full account of what has been happening:
I have two backup scripts which run on alternate days, backing up to separate local disk media sets, but otherwise they are identical. I have used them for years and occasionally they fail - in a logical way that is easily sortable, often with the help of this list. Recently each has failed to fully back up one client - it will backup the startup disk on that client (which is an external SSD) but the internal disk and another external fail with a -530 error. I can send you a screen shot if you don't believe me. Yesterday the client was upgraded from MacOS 10.11 to 10.12 and I took the opportunity to run DiskWarrior on all the disks. It found no reportable damage but in my experience it can correct directory errors that are not apparent. I then opened Retrospect and ran one of the scripts that was reporting errors. It operated perfectly, backing up all three disks on the client.
These scripts are part of a series which run every (other) day; the Engine is not shut down between the operation of each script so none of what you describe in para 3 is relevant. The script which ran perfectly when triggered manually then operated automatically later that day. It failed to find the client or any of its drives reporting -530 errors for each drive (backup client not found). Again, I can send you screenshot of the log if you need it.
All of the other clients on the script continue to be accessible to Retrospect Server and are backed up.
Any suggestions?
Thanks
Toshy