You could try removing xarchiver to see if that removes the "sketch/png" error. This way at least you'd know if it's really xarchiver causing it.

The catoriseplus triggers is perfectly fine (if you still want to get rid of that message then you'd have to uninstall catoriseplus, obviously).

The KEYEXPIRED stuff is something beyond our control, for now. Someday (hopefully) "we" (for whatever definition of "we") will have a mirror-copy of the nokia repositories, so that "we" have control over it and "others" cannot break it.

Re. CSSU. You'd have to ask the CSSU people. AFAIK going from "stable" to "testing" is possible. But I'm sure there's some post explaining precisely this. Probably just changing from one repository to the other.