Hi appreciate any help with the following.
Dell XPS 13 9343 (2015), running Win8.1 Pro, have had it some ~6 months.

A few weeks ago:
Inserting anything into USB drives after startup results in code 28 error i.e. computer recognises that there's a device attached, but can't find the relevant driver, and so can't run.
However, when booting with USB device already connected, no problems.
Same problem with flash drives, mouse, keyboard.

Have tried:
1) going into Device Manager and uninstalling all USB entries
2) disabling and enabling USB entries in UEFI
3) downgrading and upgrading BIOS (all the way from A05 to A01 and back)
4) Windows troubleshooter (always results in "can't find driver")
5) upgrading to Win10 (thankfully everything else still works)
6) updating all drivers, including for chipset
7) swear words

Seemed to coincide with installing Avira AV, and have since uninstalled.

Help!