The config has been running since over an year. So I am sure it is nothing to do with the configuration.
Log files are also rotated on daily basis. Even upgraded clamav installation, cleared olb DB with new one, etc.. There was permission issue with /var/clamav which I resolved before posting here. In between another server too faced the same issue but it mentioned out-of-memory issue in the message log file. I tried a few things but didnt work. I saw that swap space was being used. And finally I decided to restart the server. After restart that server all is well so far. I did fire fighting with the server that did not complain of memory issue. In the end I restarted that as well. That too is working fine now! :( no clue how to get over this issue without restarting! I am posting another topic to know how to deal with swapping and free swap space if memory is available.
