Ciao Nicola,
It looks like werid Joomla 4 error.
Question: Since when you have it?
Maybe you should recover website from backup from the day when everthing was OK or start from beginning.
And in the meantime ask on forum.joomla.org
If this is Joomla bug (as it looks) should be corrected by CMS update / Joomla Developers.
I have crazy idea that may help:
Use FTP and delete all languages from this folder: administrator\language
keep only those two folders:
- administrator\language\en-GB
- administrator\language\overrides