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Yep, this error came from JCE Professional's system plugin, namely its media field override functionality.

TL;DR: Always look at the error trace to see if the problem comes from somewhere else. In your case, you see /plugins/system/jcepro/src/PluginTraits/MediaFieldTrait.php Line 70. This immediately points you to the JCE Pro plugin. Even if you don't know which plugin it is, look in its folder (displayed above) and check its manifest. It tells you who you want to contact next, which might be a different developer than that of the component you saw the error in. Remember that plugin issues are not under the control of the component or its developer.

Now, you may wonder how on Earth is it possible that JCE causes a problem in Akeeba Backup as there is no media field in Akeeba Backup to begin with.

Well, Joomla! has a few “general purpose” onContent* events which fire every time there is a form loaded, prepared, and displayed to the user. Different plugins will handle these general purpose events to achieve wildly different things such as replacing content; displaying custom fields; displaying comments; adding, replacing or removing fields; etc. The way all these wildly uses cases are catered for is that every time there is a form being handled –that's about half of the admin pages– Joomla! calls the respective onContent* event handlers of all plugins, each event handler responsible to figure out when and what to do. If any of these completely unrelated plugins' event handler crashes during this chained event handling you get a stop error like the one you got. It's worth noting that neither the component the form was on, nor Joomla! itself had anything to do with the error. The error came from a plugin. That's what you should focus and address. Trying to see what's wrong with the component or Joomla! itself would be futile (“barking up the wrong tree”).

I hope this helps for any future issues you may have.