Internet sharing via WiFi won’t work (BSS stopping)

I'm trying to start internet sharing on my mac mini (share ethernet to wifi).
Until a few days ago, it worked fine, and suddenly stopped working.

I'm pretty sure my "graphical" config is fine (with the System settings app). But there's something strange in the /var/log/wifi.log.

Whenever I start the sharing, it actually works very briefly, before stopping again (MAC addresses and stuff replaced with <snip>)

2014-05-05 11:47:37.974783 AM [wifid] <en1> Start BSS on <snip>
2014-05-05 11:47:37.974984 AM [wifid] <en1> Start Network
2014-05-05 11:47:38.058925 AM [wifid] <en1> Apple SWAP IE
Raw IE: 
<snip>
Signature Length: 4
Use for Beacon
Use for Probe Response

Host model identifier: Macmini6,2
Internet Connection Sharing Enabled

2014-05-05 11:47:38.107761 AM [wifid] <en1> Stop BSS on <snip>
2014-05-05 11:47:38.117333 AM [wifid] Change Interface Role on en1 with MAC <snip> to sta
2014-05-05 11:47:38.117608 AM [wifid] <en1> Maximum PHY Rate <450>

Any idea what's wrong?