We just upgraded to v.2.6.1+ from 2.5.2 on our dev server. After also re-installing the few plug-ins we have, I began reviewing settings and doing a general scan for any potential upgrade issues. The only issue I have found so far occured when I tried to change the theme. I reset our preferences in the Essential theme, then navigated to Site administration -> Appearance -> Themes -> Theme selector. I clicked "Change Theme" next to the Default section, found Essential theme, clicked "Use Theme." The next page stated "New theme saved" and I clicked "Continue" at the bottom. Unfortunately, I returned to the theme selector page where the default theme is still standard. I have tried clearing the theme cache, purging all caches, and clearing the browser cache. This happens no matter what theme I try to choose and in multiple browsers. I have turned on debugging and do not receive any error reports that would offer direction.
Thank you in advance for any help you can offer!!