Hello, Nicolas!
First, make sure everything is up to date. Update the WordPress if needed.
According to your problem description, the error is not in the theme. Maybe some of the active plugins produce an error. Try temporarily deactivating all plugins, then check that the option works.
Anyway, there is a very easy solution: There is an “Agama options” meta box at the bottom of each post/page. On the right, open the “Breadcrumb” tab, and enter a Custom Breadcrumb title in the “Breadcrumb Page Title” box, for every particular post / Page.
https://prnt.sc/rxq1ul
Hope this helps you.
Regards