1. delete the class/smarty/plugins and class/smarty/xoops_plugins folders
2. upload the same folders from the 2.3.1 package
3. update the system module
and no success. When I cloned the default templates, I got this error message:
Notice: Only variables should be assigned by reference in file /kernel/tplfile.php line 264.
The cloned templates reflect the same number as the default set however when I view the list, there are some that are not generated.
Using the default template set, my menu looks normal.
Using the template set I just created, the left menu is extended into the rest of the page.
My apologies for the pictures, but that is the best I can do with the tools I have at hand at the moment. Hopefully this gives a bit of insight.