It seems that the "new installation" unlocked something and now everything works.
Not sure why, but glad it worked!
Now, looking at the log file appears again the message:
06/07-2015 14:31:52 IMAP#1/1: Disconnected: Connection closed by server, unexpected disconnect
But now, after some seconds (3 minutes maximum), it is followed by:
06/07-2015 14:31:57 IMAP#1/1: Connecting...
This last automatic re-connection has always successful.
Yeh, not sure why Opera wasn't initiating a reconnect before, but glad it's working, so far.