note: the only reason why the parent theme was working properly was because custom elements was not activated in the parent theme.
Thank you for looking into the problem. However that did not help. We did a complete re-install of the Theme as well of the child-theme.
Turns out the problem is not from the funtions.php or header.php. It comes from the custom elements functionality. We love that new function and we already us it.
However, as soon as the custom element function is set to: enable for “editor and admins” the problem with the tabs occurs.
When the custom element functionality is disables we can edit the tabs.
Any help on that? I assume this needs a fix in the custom element module?
Solution that worked for me provided by the developer of the calendar:
.mec-wrap dt, .mec-wrap dd {
float: none;
}