Since upgrading to 16.04, when I try to install or update a software installation in "Ubuntu Software" the password for my default admin account is not recognized/accepted. I know that the password is correct, because I can su into the account with the same password. Apparently I am not alone in this problem. I found these related threads:
but do not find the answers there helpful. In fact, only the second thread has anything I would call an answer. However, although I can handle the command-line interface if need be, I am not well-versed in Ubuntu, and have trouble finding the applications to update without the "Ubuntu Software" center. Since my failed attempts, all I see in its "Installed" and "Updates" tabs are the rotating "wait" symbols. I tried the second approach mentioned, starting "software-center" from a terminal window. The application which then starts (after producing a number of error messages in the terminal window) has the same icon as the "Ubuntu Software" which starts from my quick launch toolbar, but the GUI looks different. It too shows nothing but a rotating "wait" symbol, but I can at least identify it via "Help - About" as "Ubuntu Software Center" 15.12. The original "Ubuntu Software" from my quick launch bar has no menus which would allow me to identify it.