I have solved it myself. It was a Wireguard misconfiguration. When I logged into the router using SSH and used "ifconfig" I noticed that both routers used 192.169.196.1 as interface address. This must have caused the issue somehow. After I used 192.168.196.1/24 as "Tunnel Address" on site A and 192.168.196.2/24 on site B the issue was solved.