Silly AP
I was having problems with my wireless connectivity lately; when I sat downstairs, in the sofa, it often happened that I lost my link to the AP upstairs, only about 5 meters away from me. That isn't a normal situation, so I finally thought about setting the AP to another channel. That shouldn't do much (an AP is supposed to switch frequencies if there's too much interference), but as it's not supposed to harm either, I thought 'what the hell'. And while at it, I checked some other settings as well...
turns out one of the AP's two antennas was just switched off. Switching it on helped. Quite. And that silly setting is the default, because I had never revisited all its configuration settings after my last argument with the thing. Makes one wonder, though: why do people build an AP with two antennas, if the default settings use only one of them?
Oh well. It was a cheap thing (in fact, the cheapest one I could find), I guess you get what you ask for...