Hello,
I finally found what was the problem on my instance. It was some obselete files in templates of the boost theme, probably a site update error somewhere in the lifecycle of my Moodle site. I deleted 2 files
../theme/boost/templates/core/url_select.mustache
../theme/boost/templates/core/single_select.mustache
These template's file were calling a YUI module, moodle-core-formautosubmit which was removed since Moodle 3.3.
Removing these 2 files from the boost theme and cleaning YUI librairies from moodle-core-formautosubmit did it.
Hope this help.