Thanks for the reply.
I did not receive any errors during the update. The problem is that the folder /root/ispconfig/ was completely removed without me doing anything. Really weird. I don't understand how could this happen.
I reinstalled version ISPConfig 2.2.25 and everything seems ok now. Fortunately it did not overwrite my previous database and configuration files.
I am wondering about an attack to my system ( but would it be logical to remove ispconfig ?) What is the best tool to tracert unauthorized access ?