This is a strange Quicken bug related to accounts that have migrated to a new FID (Bank Routing #) when one bank has merged with another. When trying to import a QFX file, if the file contains already-imported transactions (when the former FID was in use), Quicken throws an OL-221-A error, when trying to import the QFX file (immediately, before any dialogs, e.g. to associate with a new or existing account). If you edit the QFX file and remove any prior transactions (STMTTRN records with a FITID that pre-existed, i.e. prior to the takeover by the new bank), the QFX file can be imported and associated etc.
The straightforward solution to this problem (avoiding the need to edit the QFX files) seems to be to select only new transactions when downloading, by date range. However that could be a real pain as you might need to do that forever for some banks, as they may by default include all prior transactions….
Overall I consider this a Quicken error as it should ignore any pre-existing STMTTRN (FITID matching) records, regardless of the FID for the QFX file overall. I also consider it a high-priority bug as it may block any users of Quicken when banks merge, and the FID changes.