You could disable SMDF for the gateway (under the gateway object) and see if this is SmartDefense related. If the error still occurs, it may be due to the protocol type & its settings for the services in the rule of the drop.
If turning off SMDF allows the traffic to pass, then there are additional SMDF checks you need to disable. In the past, with an HTTP illegal header drop I was seeing, I've found that merely disabling the check did not disable it for other SMDF checks. I had to disable all SMDF which related to the inspection of HTTP traffic. You can find that list
in this CPUG thread.
HTH