Can't connect to specific host in private network

Hi folks,

Just picked up a new 2018 MBP and immediately upgraded it to Mojave (10.14.2). It is connected to the same router as my old one (2010 MBP on High Sierra), both via WiFi. The router is at 192.168.1.1 and internet access works fine.

The odd thing is that while I still can connect without any problems to my QNAP NAS (TS-219P) with the 2010 MBP (High Sierra), be it at IP level (ping, traceroute, https for configuration) or to contact the SMB and AFP shares announced via Bonjour, none of this works on the new 2018 MBP.

From the 2018 MBP, I can ping any other host in 192.168.1.0/255.255.255.0 and vice versa. But I get timeouts for the NAS at 192.168.1.11 (the router does resolve to 192.168.1.11 when I enter "nas" which is the hostname). The same goes true for trying to access the web configuration interface of the NAS via https and for the SMB and AFP shares. For the shares, there is no difference whether I try browsing to them in Finder or try connecting with the "connect to server" dialog (e.g. smb://nas or cifs://nas or afp://nas - or the IP address). It invariably tells me that the resource is not available.

But that is not true because it still does work with the old computer and others in the same network (including another MBP also on Mojave 10.14).

I have checked and am reasonably sure that no selective access at IP level is configured in the NAS. Also, it doesn't matter whether I change the IP addresses of the NAS or the new MBP. The new MBP does not even work when it is on the same IP address as was previously the old one.

Anybody have some further ideas for troubleshooting? I have no clue where the packets are blocked. My gut feeling tells me that the only thing that changed is the new MBP, so the problem should be there. But I wouldn't know where exactly it is situated.

Thanks to anybody who might have an idea to look further into this.