And i think importing the private key in electrum is the only way that i think you can sign message.
Follow this tutorial but just follow only how to export the private key not importing to electron cash..
https://bitcointalksearch.org/topic/tutorial-export-private-key-from-blockchaininfo-for-bitcoin-cash-2063572
I hope this time you can get the private key and import it in electrum wallet download the electrum 2.9.3 but if you are running windows 10 OS you can install the latest electrum wallet which is electrum 3.0 version above http://download.electrum.org/
Thanks, I'll open a ticket to see what can be done. I've managed to find the other imported address to replace this current one until I figure out what to do.Nope, win 7 32-bit but will try some earlier versions if available.