An example of invalid characters would be &. By simply editing the .QBO file in Text Edit and removing all instances of this, it can allow QuickBooks to properly import the file.
If there are formatting issues, please contact the financial institution for assistance. The financial institution provides the .QBO file download, and they need to be made aware of this. QuickBooks follows a set of OFX guidelines set by the OFX Consortium. If a financial institution does not adhere to these guidelines, QuickBooks will not be able to import those .QBO file downloads.