When data is imported via an interface or CSV file, in some cases the process may fail regardless of pre-validation.
You can check the outcome of the transfer in the Reports section under Transfers.
Reports - Transfers view has its own tabs for reports on CSV imports for customers and products as well as accounting transfers.
Customers is the first available transfer report.
You can open the report by double-clicking the row.
On the Accounting Transfers tab, you can see the details of the outcome of the transfer of invoices and payments to accounting.
You can see more detailed information by double-clicking the row.