Hey there,
Unfortunately, I could not understand exactly what steps I should take to access your staging website, but regarding the problem that you’re experiencing it is mostly because of the old version of the theme that you were using.
Please kindly update the theme to version 5.1.1 and Cornerstone to version 2.0.6. After that please double check that you have only one instance of Cornerstone plug-in on your plugins page, if you have more please delete the old one.
Needless to say that you should have a complete backup of your website before doing so just in case.
Also, make sure that you purge all caches if you use such a plug-in and deactivate all the performance or cache related ones.
Thank you.