We finally managed to fix the existing 2.10/2.10.0.6519/latest releases by uploading a fresh build of the Linux repositories. Upon updating, apt will prompt you because of the change in the metadata once:
root@b515eca4f7fc:/# apt update
Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Hit:2 http://archive.ubuntu.com/ubuntu focal InRelease
Get:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Get:4 https://download.owncloud.com/desktop/ownCloud/stable/2.10/linux/Ubuntu_20.04 InRelease [1436 B]
Get:5 http://archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB]
Get:6 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [844 kB]
E: Repository 'https://download.owncloud.com/desktop/ownCloud/stable/2.10/linux/Ubuntu_20.04 InRelease' changed its 'Origin' value from 'https://minio.owncloud.services/clients/build-linux/4640/repo/' to ''
E: Repository 'https://download.owncloud.com/desktop/ownCloud/stable/2.10/linux/Ubuntu_20.04 InRelease' changed its 'Label' value from '4640/repo' to ''
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Do you want to accept these changes and continue updating from this repository? [y/N] y
Once you confirm the change one last time, updating will work normally again. In all builds since Jan 25, the metadata has not been included any more, so future releases should not generate such prompts any more.
Sorry for the long waiting time. We wanted to make sure everything works at last. In the unlikely event of further issues, please do not hesitate to contact us and let us know. We appreciate bug reports, especially when they are this detailed.