E10 (D3). After updating from a pre-1.9.4 version my sidebar is malfunctioning

This can happen because v1.9.4 changed the page template names. You need to edit all the pages that display the issue (the sidebar being out of place) and select the “Default Template” (or whichever other template you want to use for that page) and click Update. This will reset the page template to use the new template name.