I too had issues in Tumbleweed with this and I now typically run updates in the command line only. Seems that the applet can be a little fickle so I just don’t use it so often. I think it’d be nice if were more reliable but I seem to have better reliability with it in Leap, so long as you don’t have too many funny repos.
Tho I haven’t tested this suposition it seems like there are only issues with the applet when there is some kind of huge jump in version. For instance, most of the time you will see with
sudo zypper up
a list of packages upgraded, deleted and added. If there are packages that are getting deleted or added, the applet will not let you proceed. I could be wrong but that seems to be the case with my limited expeirence.
-Nathan