You've said that shim-signed is now installed on your system. But it's also installed at a version which is not current in any series in the archive. You listed shim-signed 1.41+15+1552672080.a4a1fbe-0ubuntu1; the current in groovy is 1.43+15+1552672080.a4a1fbe-0ubuntu2.
https:/ /bugs.launchpad .net/ubuntu/ +source/ ubuntu- release- upgrader/ +bug/1900471/ +attachment/ 5424331/ +files/ VarLogDistupgra deAptlog. txt shows a4a1fbe- 0ubuntu2 @ii gK > FU=1
MarkPurge shim:amd64 < 15+1552672080.
Again, no references to shim-signed.
You've said that shim-signed is now installed on your system. But it's also installed at a version which is not current in any series in the archive. You listed shim-signed 1.41+15+ 1552672080. a4a1fbe- 0ubuntu1; the current in groovy is 1.43+15+ 1552672080. a4a1fbe- 0ubuntu2.