For others reading this posting, part of the issue was the format of the import file. It was a CSV file and had probably been opened and edited with MS Excel. This affect the date fields.
Also, Adagio Receivables imports invoices using a 'Header/Detail' format, meaning that one row is devoted to a header, followed by a separate row for each detail. The import file used here had the header and all details on a single row, which confused the import process.
Here's the first line of the sample import invoice file mentioned above, for comparison purposes:
"I","CON01","IN5294","IN","Import invoice", "020901","NET30", "Net 30 days", "021001", "020901", 0.00, 550.00, "Reference", "AR","","", 0
"D", "CON200", "", "", 450.00, 0, 0.00
"D", "CON100", "", "", 100.00, 0, 0.00