Before reproducing, set up your instance so that there is a page with attachments which is viewable by anonymous users.

Steps to reproduce:

1. Open a new session and visit Confluence as an anonymous user (new incognito window)
2. View a page which has attachments
3. Copy the page (Tools > Copy)
4. Notice that the attachments on the page appear correctly (there is no "unknown attachment" placeholder)
5. Save the page
6. You will get an error "Failure when converting editor format to storage format."

Things to note:

• Copying a page must be the first action the anonymous user performs. If you first enter the editor by creating a blank page, then start to copy pages, you will not run into this issue
• The error will only occur for the first page which is copied. Subsequent copies of pages with attachments will not error out. But they will replace images with the "unknown attachment" placeholder
• The problem is on persistent per session. If you close the incognito window and start a new one, the state resets and you will run into the issue again on the next page with attachments which is copied