When you install Quicken on your computer, you will see QFX files described as "Quicken OFX" files. This is a great misunderstanding, as Quicken does not import OFX files at all, and imports QFX files only (also, Quicken has to be at not older than three years to import QFX files, and Quicken imports QIF files). The video below explains the confusion about QFX and OFX files.
Why? Short answer: Quicken does not import OFX files (even if it says it does).
Quicken imports QFX files (not .OFX) and calls them “Quicken OFX files.” From Quicken's point of view, Quicken should import OFX as it calls QFX files “Quicken OFX files.” In reality, Quicken imports QFX files only, and
QFX files are EXTENDED OFX files. It means QFX files follow the OFX specification and have ADDITIONAL ATTRIBUTES that Quicken looks for when it imports a QFX or an OFX file.
If you are using Quicken for PC, a great and working alternative is the QIF format. Quicken for PC imports correctly prepared QIF files fine. Quicken 2017 imports QIF files fine, Quicken versions from 2005 to 2014 import QIF files fine.
If you happened to be a Quicken for Mac (2015-2017) user, there is a possibility to import CSV Mint files (files similar to CSV files exported from Mint). CSV2CSV converts your Excel or CSV files to “CSV Mint” files.
QXF format adds to this confusion, as it looks very similar to QFX. Even more, your autocorrect may switch QXF to QFX or QFX to QXF as you type, so if you ask a support question, you may get an unexcepted answer. QXF format is not for transaction import, but for data transfer: you will replace your whole dataset in Quicken when you get a QXF file successfully imported.
Safety first: make sure to backup your Quicken file before any import: QFX, QIF, QXF, or even OFX.