There is an error while importing the forms plugin :
according to the wiki (https://wiki.ametys.org/display/AMETYSV3/Plugin+Forms), we have to add the following line at THE END of cms-ribbon-default.xml file :
<import>plugin:forms://cms-ribbon.xml</import>

It appears that if we add this line at the end (before the ending tag </ribbon>) or at the beginning (after the starting tab <ribbon ...>), it does not work. Some js files are not imported (like Layout.js for example).

The import must be done in the middle of the file (between two tab tabs)

I write it in the wiki page, and it should be modified when the issue is solved.