You should have run 'Upgrades' (admin/upgrades) after each update.

Apart from the upgrade.php script (that you can either call in the browser by the url yoursite.url/upgrade.php or execute by clicking on the "Upgrade" button on the admin dashboard page in the admin area) that only deals with the short-running upgrade tasks there is also the additonal "Upgrade" page (corresponding menu item in the "Configure" section in the admin area sidebar menu) that deals with long-running upgrade jobs. Have you executed any pending upgrades listed on this page?

If you started from Elgg 1.9 the only long-running upgrade job is likely the upgrade of discussion entries. Only if you would have upgraded from 1.8 to 1.9 there would have been more already pending on Elgg 1.9. So, just check if you see a pending upgrade on the "Upgrade" page in your admin section and execute it (it won't hurt doing a db backup before just to be on the safe side).

If you intent to upgrade further from Elgg 2.0 to 2.3 you can do that directly without first upgrading to 2.1 and 2.2. It would have been possible to upgrade from 1.12 to 2.3 directly even. One more pending upgrade should show up when upgrading to 2.2 (or 2.3) but otherwise there shouldn't be any other long-running upgrades waiting.

@iionly

Thanks for the detailed answer. I am quite sure I followed all the instructions for the upgrade procedure as stated in this page. Now and when I go to the "Upgrade" page (corresponding menu item in the "Configure" section in the admin area sidebar menu) I see a  "Your installation is up to date!" message. 

In any case I solved the problem by performing this SQL query 

UPDATE `elgg_entities` SET `subtype` = 47 WHERE `subtype` = 13;

In my particular case this updated the entities table and changed the entity subtype  from "groupforumtopic"  to "disucssion". I don't recommend it for anyone else as the number IDs will be different. 

I am sure the yoursite.url/upgrade.php  script performs a similar update function, I really don't know what might have gone wrong in my case as I ran it each time I went from version to version.

All is good now , the topics are back, so I am happy. Thanks again for answering,  you are one of the few users in this forum who provides detailed and thorough tips when it comes into solving problems.