I figured out the problem in my case. The Paypal module was not translated completly in my language (multi language ecomm) Reading the status of translation, it is not available yet in 1.6.0.11, but just in 1.6.0.9 at the moment. I fixed it translating all strings of Paypal module at least.
By the way I think that something should be improved in catching exception of missing string instead of just "payment is empty" for debugging mode...
BR