WordPress 5.6 broke the plugin

Can you try this with a default theme?

I tried it and it indeed works. Since I do not know the plugin internals, is there any clue towards where I should search for an issue? Can this one related to jQuery versions, or any other dependencies? Thank you very much.

This sounds like you’re having a different problem. Some themes install extra jQuery plugins in all of the admin pages where they can cause problems. You need to contact the creator of your theme and ask them to fix this issue.

@zgrossbart thank you for your assistance. The issue was related with a deprecated version of jQuery used within the theme, issue resolved.