I had the same problem (I expect it was due to an error during the upgrade process).
Updating /var/lib/update-manager/meta-release-lts with the lines for Hardy LTS (taken from /var/lib/update-manager/meta-release) did not help.
However, updating ~/.update-manager-core/meta-release-lts with the lines for Hardy LTS (taken from ~/.update-manager-core/meta-release, which is the same AFAI can see as /var/lib/update-manager) worked. The ~ is for the user launching "update-manager". Does it work for you ?
I had the same problem (I expect it was due to an error during the upgrade process).
Updating /var/lib/ update- manager/ meta-release- lts with the lines for Hardy LTS (taken from /var/lib/ update- manager/ meta-release) did not help.
However, updating ~/.update- manager- core/meta- release- lts with the lines for Hardy LTS (taken from ~/.update- manager- core/meta- release, which is the same AFAI can see as /var/lib/ update- manager) worked. The ~ is for the user launching "update-manager". Does it work for you ?