The Kobo Aura H2O original worked, the Libra not.
I was sure it had earlier!
Then I remembered I'd ticked a couple of boxes. Can't get in to untick when the libra is connected, so reconnected the H2O.
Unticked the two boxes one at a time.
It was the one pointed to with red arrow that causes the error if the Libra is detected.
No doubt someone will wonder why I ticked it in the first place. Seemed like something to tick!
So now everything working, reading annotations, no errors with Libra.
P.S. I searched for this error and found Dutch forum posts about some older version of Calibre or something.