When user try to load the Visual Editor in Page: pages using ?veaction=edit parameter the loading fails with the error "Error: No class registered by that name: proofread-page".
I happens because the ext.visualEditor.desktopArticleTarget.init code gets executed before ext.proofreadpage.ve.pageTarget.init and, so, the second is not able to load the ProofreadPagePageTarget
It is a follow-up of task T138489
http://en.wikisource.beta.wmflabs.org/wiki/Page:Dictionary_of_National_Biography_volume_51.djvu/384?veaction=edit reliably works for me in Chrome/Safari/Firefox…
It still happening some time. I really thing that the first description of the issue is the right one
Change 296052 had a related patch set uploaded (by Tpt):
Loads PageTarget all the time with VisualEditorPluginModules
Change 296052 merged by jenkins-bot:
Remove PageTarget.init, instead load via VisualEditorPluginModules