For the last few days, I have been missing live edit preview windows on math.SE. The same bug occurs on multiple browsers and multiple machines. It does not occur on other SE sites, e.g. matheducators, nor does it occur if I use a different IP address (I only tried one, in Europe). So, strangely, the bug appears to be IP specific.
Thanks to Martin's link, I noticed the "show preview" button I don't know how the preview ever got into a hidden state, but now it works fine after re-enabling it.
Unhiding it on one machine also unhid it on other machines (at the same IP). So it is somewhat puzzling what the scope is for this user setting, i.e. it seems to somehow be tied not only to user-accounts but also IP addresses (or at least it appears so in this case). Possibly this is a bug.
I will leave this question up since it may help others who find themselves in a similar predicament.