Draft editor content corrupted when use CTRL+Z on attachment/image in Firefox and Internet Explorer

Summary

Using a specific sequence (as explained in Steps to Reproduce) will break publish functionality in Confluence. And drafts will throw an error on publish.

Environment

• Confluence 6.4.1
• Firefox (v.56) and IE11
• Chrome 64.0.3282.140 (not all versions of chrome affected)

Steps to Reproduce

1. Open empty page in Firefox or Internet Explorer
2. Paste image that was copied from any tool (you will see upload progress popup)
3. Do a control-z to revert changes
4. Paste content again (you will see upload progress popup)
5. Publish page

Expected Results

Page will be saved

Actual Results

Page will throw the following error in logs and show a message that the page cannot be saved.

2017-11-01 12:39:12,853 WARN [http-nio-8090-exec-70] [content.render.xhtml.DefaultFormatConverter] validateAndConvertToStorageFormat XhtmlException converting editor format to storage format. Turn on debug level logging to see editor format data.  -- referer: http://confluence-base-url/pages/resumedraft.action?draftId=109020128&draftShareId=810eb33e-30d1-4b0f-8b5b-853e6e1cd5f8 | url: /rest/api/content/109020128 | traceId: a570c64579f08d75 | userName: conf.user com.atlassian.confluence.content.render.xhtml.XhtmlException: RuntimeException occurred while transforming editor format to storage format (null) 	at com.atlassian.confluence.content.render.xhtml.editor.EditorXhtmlTransformer.transform(EditorXhtmlTransformer.java:58)

Notes

• Removing the image will not fix the issue
• Issue is not reproducible in Chrome (v. 61) and Edge (v. 40)

Workaround

Create a new page and copy/paste the content from the affected page
You can also use Source Editor for Confluence if copying page body does not work