XGS 5500 Showing Invalid traffic "Could not associate packet to any connection"

I've been facing an invalid traffic issue with an XGS5500 (SFOS 19.5.2 MR-2-Build624) for a few days.
the connections of some PCs to a particular domain seem to work but then I start to log errors such as "Invalid traffic". The firewall rule is a simple LAN to Wan rule without any checks. At the same time other PCs access the same domain easily. If I change the IP of the affected PC then it starts working properly (the next day I revert to the old IP it works fine). And this is not  TCP conn. establishment idle timeout value problem.