I can confirm Gustavo's theory, after editing the files in /etc/apt/source.list.d/ and eliminating the accented characters ("actualización", for example), the entries are removable or selectable again in software-properties-gtk; so this must be a string handling bug in that program.
I can confirm Gustavo's theory, after editing the files in /etc/apt/ source. list.d/ and eliminating the accented characters ("actualización", for example), the entries are removable or selectable again in software- properties- gtk; so this must be a string handling bug in that program.