Symptoms

1. After upgrading from 3.5, the editor comes up blank for all upgraded pages and only the page title is shown
2. Newly created pages are not affected

The following appears in the atlassian-confluence.log (also thrown when re-indexing):

java.lang.UnsupportedOperationException: The body of this ContentEntityObject ('Page Title') was 'WIKI' but was expected to be 'XHTML'

or

java.lang.UnsupportedOperationException: The body of this ContentEntityObject ('Page Title') was 'BodyType:WIKI' but was expected to be 'BodyType:XHTML'

Cause

Confluence was not able to complete the content migration from WIKI to XHTML during the upgrade. This can often be due to plugins which are now incompatible with your version of Confluence.

Resolution

1. Go to Admin > Manage Add-ons
2. Update all plugins which have an update available
3. If any plugins are listed as Incompatible, remove them
4. Re-run the content migration following instruction here.
5. Search your atlassian-confluence.log for 'Wiki to XHTML Exception Report'
6. Go through the pages that are listed in the report output and remove the unknown macros from them. If the macros come from a plugin that is no longer compatible with your version of Confluence and does not have an update available, uninstall it.
7. If these are user macros, upgrade them to XHTML.
8. Re-run the content migration again