There is no problem with sasl according to the log files and I told you already that sasl is not even involved in receiving the emails! Do not change anything in the sasl files, it will just cause more problems. If the setup works on then thousands of servers with the exact files that are in the ispconfig installer, it will work on your server too. We just have to figure out where your server is configured wrong and when you change the working default files of ispconfig, we can stop right here as we will not find the problem then.
According to the mail log file, your server forwards the email to the server 188.8.131.52. Is this the IP of the local server?