npm update -g cordova installs 3.7.0, which has not yet been released and seems to be broken.
For everything to be alright, I had to
npm install -g cordova@3.6.3-0.2.13
(even if cordova platform add android@3.6.3 --usenpm seems to work, I am afraid that other things may behave strangely if we use half 3.7.0 / half 3.6.3 ...)
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