do-release-upgrade fails with ubuntu-minimal not found when mirror is set to mirror://mirrors.ubuntu.com/mirror.txt

[Bruno Pitrus]

do-release-upgrade fails with ubuntu-minimal not found when mirror is set to mirror://mirrors.ubuntu.com/mirror.txt as opposed to a specific mirror.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubuntu-release-upgrader-core 1:21.04.16
ProcVersionSignature: Ubuntu 5.11.0-26.28-generic 5.11.22
Uname: Linux 5.11.0-26-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu65.1
Architecture: amd64
CasperMD5CheckResult: unknown
CrashDB: ubuntu
Date: Fri Aug 6 09:47:57 2021
InstallationDate: Installed on 2021-06-29 (37 days ago)
InstallationMedia: Kubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to hirsute on 2021-08-06 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2021-08-06 09:47:05.067616
 Log time: 2021-08-06 09:47:26.102381
 Log time: 2021-08-06 09:47:56.742316
 Log time: 2021-08-06 09:47:58.794507