This is because debian-installer somehow was uploaded for 18.04.6 built against shim-signed 1.37~18.04.10+15.4-0ubuntu7 instead of shim-signed 1.37~18.04.11+15.4-0ubuntu9. Not sure how or why this happened, but shim-signed has been released to bionic-updates now, so I'll rebuild debian-installer for the fix.
This is because debian-installer somehow was uploaded for 18.04.6 built against shim-signed 1.37~18. 04.10+15. 4-0ubuntu7 instead of shim-signed 1.37~18. 04.11+15. 4-0ubuntu9. Not sure how or why this happened, but shim-signed has been released to bionic-updates now, so I'll rebuild debian-installer for the fix.