I had the same problem and found a solution at http://photo.net/bboard/q-and-a-fetch-msg?msg_id=0089WF
It worked for me.
There are 3 simple solutions which will not
cost you anything. I used the first method for 1+ years till I
discovered the 2nd and 3rd method just now, which should solve your
problem easily for good (hopefully).
1. After the speedlight pops up, use your hand to touch (depress softly
and let go softly) the button that normally locks the speedlight. To
find this button/protrusion, look carefully in the region from which
the light pops up. You will see a tiny black piece protruding out. This
piece matches the cavity on the back of the speedlight. So it should be
easy to find. Try pressing the button and then letting it go. If the
error message doesn't go away, try pressing it once again and then
check the monitor again. This method may take several tries.
2. A simpler solution
is to use your hand to slow down the speedlight as it pops up. The goal
is to prevent a violent shake when the speedlight unit pops open. So
use your hand to let the unit slowly come to open.
3. A more permanent fix is to put something in the camera to
cushion the shake as the speedlight pops open. If you look carefully
how the speedlight opens, you will see that there is a plastic rod, one
end of which is attached to the speedlight and the other end glides
along a track/groove as the unit opens. Just put some sticky soft
substance at the end of groove where the rod will come to rest when the
unit opens. Put such substance there so it will "permanently" cushion
the opening. One such substance you can find easily is the type of glue
many magazines use to stick ads/CDs,etc. to the pages. As far as I
know, you CANNOT get such glue easily in stores, but those used sticky
glues in magazines are good enough and you just need a tiny bit. I
happened to have bought some such glues recently from http://www.gluefast.com/catalog/product_info.php?cPath=21_27&products_id=28
for no other reason than that I have always been fascinated by such
glues. You really don't need to buy these because they come in large
quantities and you can just use the magazine glues I mentioned. (some
call these "peel and stick glue" and some call it "fugitive glue".) Yet
another possibility that should work but might not provide as much
buffering/cushioning power is to cut a tiny piece of eraser that fits
tightly into the end of the groove.
Hope these belated tips help you somehow.