just want to take a few minutes to sum up the facts... there is a lot of information here and for someone new to this it might be a bit overwhelming.
1. All AR9331 based routers suffer USB 1.1 to USB 2.0 interoperability problems.
2. when purchasing routers from a single vendor some routers fail immediately, some routers work perfectly, some routers work some of the time. For example, I have 11 alfa AP121U routers here 5 of them fail immediately 5 of them work perfectly and one of them works some of the time.
3. the problem is somehow related to WiFi. When you disable the WiFi the problem goes away.
4. The problem can be resolved in every single case by using a passive USB 2.0 hub
5. The problem only happens with USB bulk transfers. Isochronous transfers seem to be OK.
6. the problem is not a power supply issue
7. this is not related to the place of manufacture of the chipset (happens with both korean and * made chips) or version.

Actually, the chip has a hardware bug where spurious RF emissions affect the USB PLL. Of course, they will not aknowledge it as a critical bug as they found a software workaround for it.
This workaround is using some undocumented registers, and even the datasheet is normally under NDA only, which makes it very difficult for open source initiatives like OpenWrt to catch up. We basically just lost 2 years because of information obfuscation wink

you have to tweak some USB PLL bits every time you change the WiFi (init, change, reset...).
The original macro appeared publicly in QCA's github, but the merit of applying it to OpenWrt goes to the Carambola2 developers!

This may be the final word on this whole story regarding the AR9331's USB stability issue...
Following a conversation that took place between Elektra from the Village Telco and nbd from this forum, it appeared that the "AR9331 chip only implements EHCI, so the USB port is only 2.0 compliant".
The "USB 2.0" term used by Atheros (and other manufacturers too) is rather confusing, since there can be some "USB 2.0"-labeled devices which are in fact only using Full-Speed or even Low-Speed actual protocol. The "USB 2.0" sticker only says that they passed the corresponding version USB-IF compliance, but not at which speed...
At this light, I tried several configurations, and it looks like that the AR9331 only effectively implements EHCI only and not OHCI, so only High-Speed (480Mbps) devices are supported by the AR9331 chip.
So, to summarize, i strongly suspect that:
REAL USB 2.0 High-Speed 480 Mbps devices such as 3G modems or USB drives are supported by the AR9331
SO-CALLED USB2.0 or ACTUALLY USB 1.1 Full-Speed 12 Mbps such as almost all cdc-acm devices (including GSM/GPRS modems, FTDI chip-based devices, POTS modems, GPS devices, Arduino boards, etc.) appear to work but in fact feature the strange bugs we found and do not work reliably
SO-CALLED USB2.0 or ACTUALLY USB 1.1 Low-Speed 1.5 MBps such as mice, keyboards, joysticks and Software-based USB AVR implementations do not work at all
In order for such USB Full-Speed 12 Mbps or Low-Speed 1.5 MBps devices to work with the AR9331, you need to add a High-speed hub (active or passive, it doesn't matter, provided you respect the 100 mA w/o negotiation / 500 mA max current from the USB spec) in-between.
As it looks like it really is a low-level hardware limitation and not a software OHCI driver problem, in this case they seem to work (at least for days) without problem.

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