I think I understand what is happening and this may not be a bug, just confusing behavior. In text mode, music shifts to the right when you paste (in line with the text editor metaphor where words shift on paste). If a line is completely full of music (I,e notes go to the end of a line) then there is no space for the music to shift to and the system will detect this and disallow the paste. You need to delete what is there first (or select it so it deletes the selection when you paste). We will consider adding a message stating why paste was not allowed to make this more obvious. Most of our users use the default table mode which pastes destructively over existing music and don’t see this.
Dave