is right: your System is NOT up-to-date.
But, your System is non upgradable, too.
Because the needed update cannot be installed. Sorry.
Which way to resolve?
Simple: you need to remove the problematic package from your System!
What is that package?
Simple: it is named:xfce4-weather-plugin
So, open your Synaptic and search for the xfce4-weather-plugin
and remove it.
After, recharge the package-list in Synaptic. After that, close Synaptic.
Closed your Synaptic (and NOT before!), you can launch your lomanager