In the past I defined in Library--->"monitored folders" a folder which does not exist any more.
How can I remove this entry?
I miss a "delete" button.
Assuming the directory no longer exists because you removed/deleted it from the file system:
1- Using a file explorer create the directory that no longer exists EXACTLY the was it was spelled and in the same location as it was.
2- Preferences > Library > Monitored folders > Choose folders and then uncheck the directory you no longer want to see.
3- Then delete the directory you created in step 1