Thanks Adam for the reply. As I mentioned in the support ticket also, I would encourage Lime Technology to be more proactive in communicating with us around planned works and outages / service degradations. For example, https://status.unraid.net doesn't mention the current service degradation with flash backups but it most definitely should.
At least for my setup, the problem appears to be on the server (Unraid) side. I inspected:
/usr/local/emhttp/plugins/dynamix.my.servers/include/UpdateFlashBackup.php
which is the script that is executed and found it essentially runs:
git -C /boot push --set-upstream origin master
which for me fails:
root@<host>:/boot# git -C /boot push --set-upstream origin master
fatal: '~/flash.git' does not appear to be a git repository
git-annex-shell: git-shell failed
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
The same command however does work on another Unraid server I have.