I've upgraded my system from Ubuntu 11.10 to 12.04 yesterday and it gave such serious problems that I had to format the partition and re-install the OS from scratch this morning.
Now, I fixed all the main problems, but still I needed to install the old Synaptic since I need some specific software I could not find in the Software Center.
After installing Synaptic from Terminal, I was not anymore able to use nor Software Center, nor Synaptic. They open and then close immediately without any message. The only thing Synaptic says is (sorry: I0m translating the message from Italian):
E: Line 59 in /etc/apt/sources.list is not correct (dist parse) E: impossible to read the sources list. Check the problem in the repository configuration. E:_cache->open(), failed, please report.
What should I do? Thank you every body for your help!