@jessicadunbar I’m not sure I fully understand your reply, so here goes what I can tell you:
Replying to your first paragraph: after having edited the native Concrete “Image block” (replacing an image), the error Access Denied happens after every page load. That means browsing the website while logged in as a user with admin permissions. It’s been 4 years since an image block has been last touched upon, and it seems it’s part of an override in the application folder. Check the following thread:
Second paragraph: Thread for Concrete CMS site’s configuration 9.1.2
Edit: I’ve tested this on a different server, upgrading from 8.5.9 to 9.1.2, adding a content block did the same thing