try this for another route...
under Menu > Administration > click on "Synaptic Package Manager"
When that loads, (after entering your password) click on "Reload" (top left button). When it updates the repositories, then "Mark All Upgrades", then if any appear, click on "Apply"
I go this route sometimes when I, too, have trouble with mintupdate.
Malachi 3:10 NRSV
...says the LORD of hosts... I will not open...windows...for you...
Linux user since 2001 pda; 2006 lappy