TCP i/o timeout Syncthing

[babayusa]

I am running syncthing on two PC's (one WLAN, one Ethernet) and a mobile phone.
It works fine on the WLAN PC and the mobile phone.

Local discovery doesn't on the LAN PC.
The two devices can ping each other, they discover each other in Syncthing but I get a TCP i/o timeout error on both devices. I copied almost all Networking Support Kernel options from the Laptop to the PC but it still doesn't work on the PC.
When I netstat -aut I can see a Established for not more than a few seconds, mostly just SYN_SENT and SYN_RECV on both ends. Sometimes FIN_WAIT1 on the WIFI PC.

I think this PC is closing the TCP connection almost immediately but I don't know why. There is no firewall in place.
Maybe this isn't a gentoo problem.

Kernel config: http://0x0.st/XJqF.txt