Intrusion prevention - System is clear

Hello,

for a few days now, one of our servers is beeing attacked on Port 443, or at least what SEP says:

Name of IPS:

Attack: an intrusion attempt was blocked.

Status:

Blocked

Signature:

System Infected: Trojan.Naid Activity 2

Attacked:

System

Attacked Port:

443

We already checked the registry, for the files and the services like it is written here, but we can't find anything on the system. The services AppMgmt and BITS do exist, but they are disabled/set to manual. A full scan isn't able to find anything suspicious on the machine either.

Any suggestions or ideas on how to fix this?