I've found a solution here:
http://stackoverflow.com/questions/7476420/gwt-blobstore-error-calling-createuploadurl
Just use your computer name in dev mode instead of 127.0.0.1. In FF
you will be asked for adding this name to GWT dev plugin automaticly
and in Chrome you must do it yourself (as it is described at
stackoverflow), I dont't know about IE.
I'm developing my app quite a while and I didn't have this problem
before. Some lib upgrade (GAE/GWT/GPE ?) to the new version must had
caused it.
I'm wondering, since I didn't try my app on the production, if there
will be some cross-domain problems when the app is deployed on GAE and
is using Blobstore URLs...
Regards,
Krzysztof