We were having same issues. Some we managed to get working again by removing dated plugins but others not. Hired a js professional via Upworks and he fixed my issues
This was our root problem with Mootools-core being outdated from Joomla.
His fix for us is:

1. locate mootools-core in media/system/js folder
2. open mootool-core
3. Search for a?[]:f.isEnumerable(a)&&"string"!=typeof a?"array"==b(a)?a:e.call(a):[a]}
4. Remove
5. Save

Plus d'onfmroation sur les sideeffect on :
https://forum.joomla.org/viewtopic.php?t=989407&start=30

Icic un plugin a installer permetttant de desactiver la bibliotheque Mootools
https://digitaldisseny.com/en/joomla-extensions/disable-mootools-joomla-per-itemid-solve-jquery-conflicts