I had the same problem and solved it with deactivating all plugins and then reactivating one by one until I found the one creating the issue and replaced it with a substitute. In my case an out-dated contact form plugin. Alternatively you could try to downgrade that particular plugin if you are sure it worked before.
I currently have this same issue. I've downgraded to 4.9.9, disabled every single plug-in, and uninstalled and reinstalled Oxygen. I've tried reverting Oxygen to 2.1, then upgraded back to 2.1.2 - all to no avail! Still unable to edit pages or templates. The editor also will not trigger if I create a new test page - but it doesn't produce an error... just a continual spinning gear.
I used shortcodes directly as text-boxes within some of my templates. For some shortcodes, this made the templates un-editable with Oxygen - and I got said error. The fix is to disable said plugins so that they don't render (try disabling all plugins except Oxygen, for instance), and then put your shortcodes inside actual shortcode-elements within Oxygen.
It's a weird error, and I suspect that it collides with other elements within Oxygen. Dunno which exactly.