Let me describe briefly how a few problems arose while I tried to upgrade from 12.9 to 15.2.
During the initial tiki-installer section, at the point we connect the domain to the database, I got the following error: Fatal error: Uncaught exception 'TikiDb_Exception_DuplicateEntry' with message 'Duplicate entry found (...) Exception.php:13 Stack trace: #0 /var/www/vhosts/mydomain/mysubdomain/lib/core/TikiDb.php(164): TikiDb_Exception::classify('Duplicate entry...') #1 (...) mysubdomain/lib/core/TikiDb.php(72): TikiDb_Pdo->query('UPDATE `tiki_pa...', Array, -1, -1, 'exception') (...) Exception.php on line 13
Despite this, my wiki looked like it was working properly and I actually worked on it for a couple of days. Then, while playing around with the look&feel, I noticed another error message, saying: Database Version Problem Your database requires an update to match the current Tiki version. Please proceed to the installer. Using Tiki with an incorrect database version usually provokes errors. If you have shell (SSH) access, you can also use the following, on the command line, from the root of your Tiki installation: php console.php database:update.
Well, I didn't have secure shell access, so I asked the hosting company to run php console.php database:update which not only didn't take care of the error, but actually made me lose admin privileges. I had to go back to my 12.9 installation (through restore) to have my wiki operational again.
So my question is: What went wrong? What the above two messages indicate and how can I safely upgrade to 15.2? (well, three questions actually)