I upgraded to version 7.2.3 from a 6.8.7 version, and now the sidebar doesn't work properly in my Mystique skin (the latest 2.1.3)
There is a set of buttons on the right that show categories, tags, etc.
Now when I click on one of them I get the error page 403 forbidden which is simply my 403 error page.
I've found the location of the anchor #instance-sidebartabswidget-section-categories in the skin.
General Support
b2evolution CMS Support Forums
Your host may be using Mod Security and the new version, 7.2.3, may have triggered the 403. If have access to your host try disabling Mod Security and see if that helps.
Also check your php version.