> What is your main conection (WAN side) ?
 > If wifi, check the quility of signal. This is happend when are some 
packet
 > "loos" and AP is trying to correct them. (depends on AP)
 > peter

Cable, no wifi

 > Have you tried to allow fragmented packets (default pass rule on LAN
 > interface)? Have you tried to lower the MTU of the WAN interface?

Tried both, didn't make a difference

 > ...one more example with same problem:
 > If you are using proxy server this could happend. I used to use the
 > 602LanSuite,Very good proxy sever but with some version of Internet 
Explorer
 > I had same problem. As soonm as I installed a mozilla browser 
everithings is
 > OK. Try diferent browser, this looks like problem at Your side...

Not using a proxy here, and since all browsers on my network dies, I 
doubt it's a client-issue

It looks like it has something to do with the traffic shaper, since the 
error hasn't appeared since I disabled it. No, I don't have any rules 
for HTTP-traffic