All my .csv files have suddenly turned into a single line of Japanese characters. What has happened?
I now have the answer. The imported data was in the UTF-16 character set. This must have changed when LibreOffice 6.0.1.1 was downloaded, despite it having been OK with the previous version. Changing to UTF-8 cured the problem.
More likely UTF-16 was selected once for a different import, which may happen automatically if the file content was prefixed with a Unicode UTF-16-BOM. The selection is remembered in the user configuration profile to be presented for the next CSV import.