Unresolvable xrefs since lates update?

Since the latest update of FM2019 one of my documents has an issue with its xrefs.

Tried to update them - the pod says fine - the console on publish says no.

Tried to remove them and inserting them again.

Pod says yay fine ... console on publish says no.

Now am frustrated since all other refs work and just that one not.

No errors on opening the referenced files... nothing that could indicate that behaviour.

And its not the first time I am using this funcitonality and in all other docs it is just fine.

Checked the xml physically also outside FM to see if IDs or something might trigger the issue and all looks good.

Evend did a gazillion times the DITA update references ... nothing.

Any ideas?