I don't know if you still want this info as I have successfully upgraded via the command line do-release-upgrade. Let me know if you do.
I think I narrowed the bug. It is an interface bug. The GUI update-manager reports disk space problems, but not the partition which has the problem. I kept deleting stuff from '/' but the problem, as reported by do-release-upgrade, was in '/boot'. I got rid of some really old kernels, and the upgrade proceeded.
I don't know if you still want this info as I have successfully upgraded via the command line do-release-upgrade. Let me know if you do.
I think I narrowed the bug. It is an interface bug. The GUI update-manager reports disk space problems, but not the partition which has the problem. I kept deleting stuff from '/' but the problem, as reported by do-release-upgrade, was in '/boot'. I got rid of some really old kernels, and the upgrade proceeded.