I am using xubuntu, but I am pretty sure that other variants are affected too.
My problem started with being unable to start synaptic from the menu.
So far, I have found out that synaptic is started with pkexec, the successor of gksu.
When I test starting
in a terminal, it asks for the password in text mode.
According to the pkexec man page, there should be some authentication agent registered somewhere.
With another test
in the terminal, it says it has not found any authentication agent.
pkexec --disable-internal-agent /usr/sbin/synaptic
So, the question is: where is the authentication agent registered ?
Thanks and Regards,