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    This has been a very useful thread.
I am using SMARTSECTION 2.12 RC1 (and now XOOPS 2.0.16) and the notification.php page was blank, but not for all users.
The debug showed this:
 SELECT * FROM prefix__modules WHERE dirname = 'smartsection' 
SELECT * FROM prefix__config WHERE (conf_modid = '2') ORDER BY conf_order ASC 
SELECT title, short_url FROM prefix__smartsection_item WHERE itemid = 170 
Error number: 1146 
Error message: Table 'database.prefix__smartsection_item' doesn't exist  
N.B. smartsection_item
s is a table name.
The content of prefix_xoopsnotifications included this:
 52 |         2 |        170 | item          | approved         |       1 |        1 | 
|     53 |         2 |        185 | item          | approved         |       3 |        1 |  
(There was also another one for the admin, that he be notified when article approved.)
I logged in as admin and deselected the "notify when articles approved" option.
I used sql to get rid of the rows above (delete from x__xoopsnotifications where not_id=52; delete from x__xoopsnotifications where not_id=53;)
Now it works fine. 
I don't really need to have the "notify on approval" option, so just won't use it any more. I am assuming (perhaps wrongly) that that was causing the problem.
Perhaps this will help someone.