kolev: If you've upgraded to LEM 5.7, there's a Hotfix 1 that specifically relates to the Log4Net connector.
From the ReadMe:
Agent Issue Resolved
• This resolves support case 535995 - Unable to start/stop connector instances
- This only needs to be done if using the "Log4Net" connector
• This resolves support case 268630 - Unable to report Nessus related events
- This only needs to be done if using the "Nessus Security Scanner NBE Report" connector
You may want to deploy the Agent fix (you can get it from the customer portal) and see if that resolves the issue.