this issue usually happens when the Server timeout is pretty small but we have to differ between uploads and downloads here, because this is a really bit difference:
For downloads it should be suitable to enable Progress Indication Methods/Data Trickling to prevent the connection to time out. In this case while the download from the server is running some of the data is forwarded to the Client, which ensures both MWG and Client do receive some data and there is no idle connection.
If you do not have Data Trickling enabled, data is tranferred from the FTP Server to MWG and is filtered there, which takes some time. There is no data between Client and MWG therefore the FTP Client will close the connection.
For Uploads this is more difficult:
If you upload a file the FTP Client sends a PUT command to MWG and this is immediatly sent to the Server as weill. While you are uploading from your Client to MWG and filtering on MWG there is no data being sent to the server, although we told him that we are going to send something. If there is no data on the idle connection the server will close the connecion and the upload will fail.
Unfortunately the latest MWG 7.x does NOT support Data Trickling in the FTP Upload... this should be in the product in the near future since I have filed a Feature Request for this as it is really important for Uploads.
So to have big Uploads work through MWG the following must be ensured:
Download: Have a Rule Set in place that enables Data Trickling
Upload: Ensure that no progress Indication is required, e.g. MWG will forward the file straightaway with no AV filtering
As an alternative you can play with the Timeout settings on the Client and rely on relaxed timeouts on the server.
If this does not help we should gather some feedbacks and packet captures which we should look at in support.