Thanks PGR,
but no, this was not a front-end error.
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Many of the accounts to import had “phone_alternate” with a prefix 'Alt Phone: '. That invalid prefix for phones was not validated and did not throw errors during the import.
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The accounts had email addresses but no “primary_address” field because it was not present in the example csv file downloaded via the import dialog in SuiteCRM. It seems that an account with a single email address assigned that is not flagged as primary causes this validation error as well.
The import routine should have flagged the relevant email addresses as primary.
All imported accounts are valid now.