Steps to reproduce:
1) Open any table containing columns of DATE type, go to "Data" tab
2) Find any non-null cell in said column
3) Select said cell, press shift+delete. Cell's value becomes (null) and row is marked for update.
4) Press Commit button or F11
After that, this line appears in "Messages" window:
One error saving changes to table "SCHEMA"."TABLE_NAME":
If you clear column by typing space and deleting it, cell's value becomes (null) as well, but commit works. Update statement from worksheet also work, so it's not a table issue.