Though a new problem arises, I can't sign from the electrum wallet :/
That's because there is currently no "standard" solution for signing messages using SegWit addresses... so you can't sign messages using any SegWit wallet.
Or are you talking about signing transactions?
Both. I found out indeed that there is no way to sign a message yet for segwit addresses, and, like what you said, there is no standard yet to sign messages for segwit addresses.
However, the problem is on the new Electrum 3.0, I can't even sign transactions with either segwit addresses or even legacy addresses. Not sure why is that, but I'll try to use the old versions and see if that fixes the problem.