The TwoColConflict extension is overriding EditPage for all ContentModel. So, it is overriding the custom EditPage subclasses used by some ContentModels like the "Page" and "Index" one of ProofreadPage, making them not possible to edit.
The "AlternateEdit" hook handler should be modified in order to don't do anything on contents that are not Wikitext.
Examples of affected pages when the TwoColConflict beta feature is enabled: