Hello Stefan,
Have you enabled the adapter of your receiver channel for virus scan? Please find the screenshot for where you can do it -
And also you need to choose the virus scan option in your Receiver agreement. Sorry, i do not have a dual-stack hence cannot provide you with the receiver agreement screenshot, but here is a screenshot of ICO -
You can refer to the below blog from Markus for more info on the same -
SAP NetWeaver PI 7.31 - virus scanning - example setup
Regards,
Nitin