It appears this bug re-emerged (already reported with CONF-11508 and CONF-12356).

1) Insert a new table.
2) Insert a new table row via the appropriate icon in the table toolbar (or keyboard shortcut Alt+Cursor Down)
3) the new row has a space character in every cell.

Ceteris paribus with new columns.

The obsolete space character is preserved when saving the page. It is particularly annoying with the new sorting function (by clicking at the table header), since the space character sabotages the expected way of sorting.