Brion:
Why are you sending wpPreview=0? The presence of wpPreview will engage preview mode; don't send it at all.
(Processing of save vs preview mode has changed a bit from how it used to be in changes unrelated to wpEditToken.)
In a first attempt to resolve the problems, I thought that I now had to send all elements of the form, so I gave them all values...
After removing it, I don't get any preview pages any more, however, I am now getting "500 Internal server error" error messages. I'll be checking again tomorrow, but for 'normal' editing, Wikipedia seems to be rather well-behaved at the moment.
Andre Engels