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The radix character (separator between seconds and fractional seconds) is a comma because my NLS_TERRITORY setting in SQL Dev is e.g. 'FRANCE'. If I had 'AMERICA' it would be a period.
The format mask, on the other hand, has the 'X' for the radix character, which would take the local setting. However, it ignores the local setting and seems to use the database setting. If that is a territory using the period then the insert will fail.
So it is important to know what the database setting is before doing the export. This could easily be avoided by not using the 'X' in the format mask, but in both the literal and the format mask using a hardcoded radix character.