I'd moved the My Documents folder to a location on my D drive and renamed it to Documents.
For the third time now, Windows Update has caused the folder name to be reverted to My Documents, causing apps that access Documents directly on the D drive to fail to locate the folder (I know that they shouldn't access it directly, but some just do and there's nowt I can do about that).
The solution was simple enough (i.e. rename it back again).
As explained earlier, I've just started this thread in case others see a similar phenomenon and wonder what has caused it and what to do about it.