How many threads do you have configured for AV scanning? The value might be too high. Check your advanced settings.
See this topic: https://community.mcafee.com/message/243797
I'm not sure how much liberty you have for testing but I'd be curious to know if this problem persists if you were to temporarily change the anti-malware ruleset so that it does not scan in embedded objects. Could it be that a user or process is downloading a type of archive that has multiple layers of compression and expands to an extremely large size?
It seems that a misconfigured Linux server was causing the problem. The server did open more than 80 connections to the webgateway in parallel and was starting then downloading.iso files from an ftp server. I blocked this traffic and now everything went back to normal.
This feedback was sent by McAfee support (thanks!). Anybody an idea how to find this issue by myself next time? Other than doing a netstat on console?
Thanks again for your support!