To help people who would like to do the same thing as we did :
We had the version 11.2SR12 runing for months with good results.
We decided to upgrade to 12.0 SR1, the latest version available which had been released for a few weeks.
The upgrade was as simple as before but we soon discovered that the 12.0 SR1 had a bug in the log generation, and with the Left Join. It was impossible to keep it.
We made the mistake to try to switch to 12.0 IR instead of going back to 11.2SR12.
To downgrade to 12.0IR, we just had to launch the 12.0 SR1 installation file, then choose Uninstall and Relaunch the installation with 12.0 IR. Every settings were kept. And it worked.
But we discovered a lot of other bugs that were fixed in 12.0 SR1 !! We could not keep the v12 on our server.
So we decided to switch back to 11.2SR12, the last stable version of Qlikview at this time (and still now I think)
The problem is that it was not as easy as before.
We uninstalled the 12.0IR, then launched the installation of 11.2SR12. Everything run fine but the Management Service did not start... There was a strange error. The Distribution Service was not good too.
I think the v12 changed settings in the Management Service files and this new configuration was not correct for v11.
We had to stop services, restore the Management Service and Distribution Service folders (delete the new ones and copy the old ones) and restart the services. Everything was ok after that. All tasks ready and all services working.
It was a bit difficult but possible without losing anything. Just don't forget to have a backup of all data before making any upgrade or downgrade.