If you didn't place   in your Preserve Entities list yourself. Then the only answer is that you deleted your Sigil Preferences at some point and allowed them to be re-created with the default values. The default entry in the Preserve Entities list was changed to   recently. We did it precisely
because it ensures being able to open all EPUB2s and EPUB3s without generating errors. If you try to open an EPUB3 containing no-break spaces with in the Preserve Entities list, there will immediately be errors. We thought it best not to subject new users to that possibility.
Anyway ... now you now. Put your back in your list and live happy (it would still be there had your prefs not been wiped/reset at some point). Just don't come crying when an unknown epub you open starts giving you pink errors!
