Thanks for your reply Gerhard
yes I have configured the switch with ' ip authorized-managers' in order to grant manager access from my workstation and it has always worked fine until just now.
I have not modified anything in the config of the switch but now it's impossible to access the switch in manager mode. I get always the console in operator mode!
Consider that I' m connecting to the switch trough SSH protocol with puTTY client, not telnet.
So, after the first login attempt (I am in operator mode) , I leave the session opened and launch another separate session with puTTY to the same switch (this was what I meant). Magically now the switch give me the manager prompt.
In addition I noticed that also the serial port does not work anymore.
This problem is strange because it happened at the same time on three switches in our network.
The switches are all the same model (5300xl) and have the same software version E.11.10.
thanks for your support,
Tommaso