Hmm, after further investigation, I do believe it is a proxy-related problem (see Bug #24250), not directly connected to apt-cacher.
In fact, if I use a syntax like the following for all the entries in /etc/apt/sources.list:
deb http://hostname:3142/it.archive.ubuntu.com/ubuntu/ feisty main
where 'hostname' is the name of the apt-cacher server, the upgrade to gutsy starts and the channels are correctly updated to gutsy, like in the following:
deb http://hostname:3142/it.archive.ubuntu.com/ubuntu/ gutsy main
This does not fix the issue but, at least, it is a workaround to perform the upgrade using apt-cacher.
Hmm, after further investigation, I do believe it is a proxy-related problem (see Bug #24250), not directly connected to apt-cacher.
In fact, if I use a syntax like the following for all the entries in /etc/apt/ sources. list:
deb http:// hostname: 3142/it. archive. ubuntu. com/ubuntu/ feisty main
where 'hostname' is the name of the apt-cacher server, the upgrade to gutsy starts and the channels are correctly updated to gutsy, like in the following:
deb http:// hostname: 3142/it. archive. ubuntu. com/ubuntu/ gutsy main
This does not fix the issue but, at least, it is a workaround to perform the upgrade using apt-cacher.