Source button in inline wysiwyg editor missing after update

We are currently updating from 13.10.6 to 15.10.11.
In 13.10.6 it looks like:


In 15.10.11 like this:

We are missing the source code button.

Any idea where it’s gone?
Simpel

PS: The button is flashing for a short time and vanishing quick but only if clicking an-edit-button-pencil right of a heading.

Hello, my guess is that you’re now using the realtime wysiwyg editor unknowingly (and it’s currently missing a source button - will be in the next release). This is likely caused by the fact that you’ve customized the list of plugins for the wysiwyg editor. See https://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/15.10.9/#HRealtimeWYSIWYGEditor