Collection tree installing

I’m writing about the site
I am attaching the error message when I activate the “Collection Tree” plugin.
Could you help me understand the problem and suggest a possible solution?
Thank you and best regards
Andrea Montagner

By error message, do you mean the text in the dropdown (well, drop-up I guess in this case) on the left side? It’s not really an error but an issue where the titles of the collections have HTML in them.

It’s just activating/deactivating Collection Tree that causes this? This almost looks more like Search by Metadata but that doesn’t affect collections.