Researched again. I am not the only one, who has problem with OH3-binding :
https://community.openhab.org/t/modbus-binding-not-working-on-oh3/111619
https://github.com/openhab/openhab-core/pull/2272
I have a zillion small ideas/tweaks. But no smoking gun
(thanks for your very interesting, educationg points about modbus)
But for noew. it is sooooo much easier to turn to planB:
- use OH2, that works like a charm (at least for modbus)
- wait for updated modbus binding on OH3.1 stable release
I see they just installed a fix in snapshot version (not in 3.1.M3 that i tried)