Actually, I did a new custom build from trunk, but including mwan3 2.9.0 instead of 2.10.2.
And I explicitly included package rpcd (without options). And, voila,
no more error messages:
mwan3 status
Interface status:
interface wan is offline and tracking is down
interface wwan is online 03h:42m:10s, uptime 03h:42m:12s and tracking is active
interface wifiwan is online 03h:42m:43s, uptime 03h:42m:45s and tracking is active
Current ipv4 policies:
standard:
wifiwan (100%)
Which looks very fine for me.
So, my suspicion was correct, that there is a missing dependency to rpcd.
Having made the expirience, that (my ?) bug reports are taken care of months (at least) after submission, I refrain to do so.
It might be much more effective, you link Florian to this post.