In the end I didn't have time to go to MediaMarkt or the Phone House, but I did go to the local Vodafone shop to get a second stick (a Huawei-made K3565-H for 29.90). I played dumb, but insisted on a Huawei stick, claiming to have read on the Internet that they had fewer problems.
It looked at first like it was simlocked (aargh!) and the firmware updater linked to above didn't work (but a look with the DC Lock client showed it wasn't), but I went to the Huawei website (
http://www.huawei.com/mobileweb/en/d...ype=-1&id=5840) and found the E160G updater. Seems to be working so far.