Looking at the main.log, this seems to be the problem:
2019-10-08 14:12:33,569 DEBUG need_server_mode(): run in 'desktop' mode, (because of pkg 'ubuntu-desktop')
2019-10-08 14:12:33,569 ERROR No 'ubuntu-minimal' available/downloadable after sources.list rewrite+update
2019-10-08 14:12:33,569 ERROR 'ubuntu-minimal' was not downloadable
2019-10-08 14:12:33,573 DEBUG abort called
This might mean either a networking error, or something being wrong with sources.list. Could you please provide the sources.list from the affected system?
Anyway, this doesn't seem to be a regression introduced by the changes currently in disco-proposed - those only touched snap-migration handling.
Looking at the main.log, this seems to be the problem:
2019-10-08 14:12:33,569 DEBUG need_server_mode(): run in 'desktop' mode, (because of pkg 'ubuntu-desktop') downloadable after sources.list rewrite+update
2019-10-08 14:12:33,569 ERROR No 'ubuntu-minimal' available/
2019-10-08 14:12:33,569 ERROR 'ubuntu-minimal' was not downloadable
2019-10-08 14:12:33,573 DEBUG abort called
This might mean either a networking error, or something being wrong with sources.list. Could you please provide the sources.list from the affected system?
Anyway, this doesn't seem to be a regression introduced by the changes currently in disco-proposed - those only touched snap-migration handling.