This was caused by an incompatible API change in update-manager:
See LP: #1026257 which unfortunately I can't seem to link to above.
I'm not sure whether it's worth fixing u-m in the way I describe in that bug report, but this could potentially affect other subclasses of InstallBackend, so it might be worth it. In the meantime, I have a hack for apturl to work around the API change.
This was caused by an incompatible API change in update-manager:
See LP: #1026257 which unfortunately I can't seem to link to above.
I'm not sure whether it's worth fixing u-m in the way I describe in that bug report, but this could potentially affect other subclasses of InstallBackend, so it might be worth it. In the meantime, I have a hack for apturl to work around the API change.