As your fault code reader is a stand alone unit it cannot suffer from USB <-> RS232 conversion timing issues

The background with Ebay cables:
Cheap "VAG COM" compatible cables from China quite often have cheap (a/k/a forged) FDTI USB-RS232 adapter chips which come with sh!t drivers... since Windows is poor with USB real time keeping it needs some good drivers and authentic chips, otherwise you won't get a connection.
I have built my own RS232 VAG interface (with a kit from ELV Germany, product code VDA100) and use a Brainboxes USB<->RS232 adapter to connect to it. Brainboxes converters are very good and made in the UK, so you can actually get support for them and they come with life time warranty... We use them at work for high tech devices.