Checking the sharepoint logs (ULS) on both servers i found the following error : "Job for import of distribution lists failed. Reason: Cannot get the crawler status --> System.Net.WebException: The remote server returned an error: (502) Bad Gateway ..." So, there is a problem with the communication between the front end server, the crawler server, and the proxy inbetween.
After some "googling" I found a solution that solved my problem on Vince's SharePoint Blog.
What I did was:
- Log on to the front end server with the same credentials as the Window SharePoint Service Timer Job
- Open Internet Explorer (IE)
- Go to internet Options Connections LAN settings
- Check the box "Bypass proxy server for local addresses"
- Close IE
- Restart the Window SharePoint Services Timer job (sptimer3).