As a radioamateur would say: A short between the earphones...
During the afternoon I tried all sorts of things. One of the points I noticed was that there was no response to ARP requests. So I ended up setting a static ARP entry on my PC. Did not work, and I finally upgraded the firmware.
When I deleted the ARP entry, things worked fine.
The problem I was trying to solve is a bit more complex - and I was simplifying as much as I could.
The main purpose for this Mikrotik is act as a flownet server. It sits inline - just before our final router and reports traffic to a raspberry PI on the x.y.z.0/24 network. The problem is that this router still does not respond to pings, and I cannot connect Winbox to it.
This router also does not respond to ARP requests.
I'll do some more testing on the test system tomorrow, and replace it in the field. Possibly the firmware upgrade?
Thanks for your help!