Twiki headings syntax not working

On XWiki 10.10 with TWiki Syntax 1.0 extension installed and enabled. CKEditor 1.29 used.

Steps to reproduce:

• Create a new page
• Switch to TWiki Syntax 1.0
• Switch to source mode
• Enter :

  ---+++ Header1

• Switch to WYSIWYG

Obtained result:

--+++ Header1-

Expected result:

  h1. Header1

When the page is saved, it is well displayed. When reopening the editor after it's saved, CKEditor failed to load properly (see screenshot).