Just for reference, Chris let me know why this was happening:
“I found the error: the input file has a column ‘lon’, but it should be ‘lng’.
Because of this difference, MapPress doesn’t realize there’s already a lat/lng, and tries to geocode each POI with Nominatim. Nominatim is very slow, so this eventually exceeds the PHP maximum runtime (max_execution_time in php.ini defaults to 120 seconds). PHP then throws an error, and you see the “Invalid JSON” message from MapPress.”
He is fixing it in the next release (making ‘lon’ a synonym for ‘lng’) but for now I have renamed the column and the import works perfectly.
Charlie