I sent my last transaction with a low fee and the "replace by fee" option enabled, just to give it a try. Now when I right click the unconfirmed tx in the History tab and select "Increase fee", it gives me the option to set a higher fee. But when I try to sign the new transaction, I get an error message that looks like this: http://imgur.com/a/O1vyD

Should this feature work when using Electrum with a hardware wallet? I did not click the "Final" checkbox after setting the higher fee; is that required? I'd welcome any help!

EDIT:  Might the error have anything to do with the original transaction only having one output?