Comment 2 for bug 1706416

The error in your log is:

debconf: (Cannot connect to /tmp/aptdaemon-za2d4hr3/debconf.socket: Connection refused at (eval 17) line 3.)

This appears to be a duplicate of bug #1694513. You will want to run 'sudo dpkg --configure -a' from a console to complete the configuration of shim-signed.