Answer
Check the actual arithmetic in the source bank statement file itself before assuming an IFS-side problem — in a confirmed case, the sum of the opening balances across the statement's detail lines simply didn't equal the closing balance stated on the header line, which is exactly what this validation checks for. Correcting the header's closing balance value in the source file (or getting a corrected statement file from the bank) to genuinely match the sum of the detail lines, then re-importing, resolved the TransferError — the error is a legitimate data-integrity check catching a real inconsistency in the imported file, not a bug.