Currently our site is editable and viewable only by us two admins. My co-author is using Internet Explorer 11. When she opened pages that I had saved in WYSIWYG mode and clicked the icon to switch to Wiki mode, she got the ajax error.
When she opened a page that I had previously saved in Wiki mode, there was no error.
Here's what she wrote about her workaround:
Ajax Error - hope my memory serves. I got an ajax error when I went to edit a page and it came up in WISIWIG and I clicked on the pencil to change it to WIKI
My workaround might not be hugely helpful unless for a user who is trying to edit pages that were in their history and saved as WIKI pages. Because, I went back to the Wiki version in History, and rolled back to that version - which I could use and save as a WIKI for further editing.
Is this what you were looking for?
I don't seem to have had the problem for a while, but I think that's because you're now the last edit on most and have been careful to save as WIKI edits.
Edited to add:
As far as the type of content is concerned, it was just text and one or two simple FancyTables.