iMac startup freeze after update to OsX El Capitan (II)

Hello,

After I installed the OsX El Capitan update on my iMac 27" late 2013 (1TB hard-disk), it systematically freezes at startup, with the black screen and the apple visible but absolutely no progress in the horizontal bar below. Only solution is to re-install the OsX El Capitan update again from scratch. This happens systematically, always the same (so, after re-installing, next time I'll shut down the iMac won't start again and new re-installation is necessary).

I have performed a HD check and it was OK.

Now the computer won't even start on safe mode. There seems to be a pattern indicating that immediate start after shutdown might be possible, however trying to start after e.g. overnight shutdown does not work.

Brief specs of the iMac below:

Thanks for your help in solving the problem.