Somehow, I think it's fixed. I didn't change anything, except some settings that I later changed back.
The element parsing bug may be gone, but since there is no actual fix, I don't know yet. I can't solve problems when I don't see where the problem is.
I've managed to find a way to change the editor, which had a different name when I looked at the problem in the past. For some reason it is not displaying correctly and some buttons don't work. If I can fix it maybe we can use the old one again.
This is where you can find the setting needed. It's by finding the Kunena > Templates tab, under Components.
The textarea is messed up and clicking Video or Emoticons makes the page dark, but the is no dropdown modal. Somehow they worked at some point but the buttons were not working... very strange. I haven't seen anyone online reporting this bug either.
Found an interesting usage for the CKEditor we are using now. When you style the text using the buttons, you can
grab it by clicking the elements that pop up on the editor footer.
I'll spend a few more hours trying to fix things... but now I need some rest.
!!! Also try to clear the browser cookies. This may be one of the problems that create the parsing problems when changing between preview and Source mode.