Re: CDS (current Environnement) Update record Error when updating a relationship.
Hello Stephan,
I hope you're well. I've had the same error with the Common Data Service (Current Environment) connector. My flow was when an Opportunity is updated, it created a new Task and assigned it to the Opportunity.
Flow
On the Common Data Service (Current Environment) action, I used "Create a new record" to create my Task record. The action has a field called "Regarding Opportunities". In that field I selected, from the "Dynamic content" pane, the Opportunity field called "Opportunity", which is the unique identifier of the Opportunity. See below:

Error
Yet I received this error.

Solution
What I had to do is add a little bit more to the field called "Regarding Opportunities", before I added the dynamic content I wrote opportunities/ then I added the dynamic content value. See below:

No error now, but a success
And as you can see, this now didn't fail, it was a success and associated my new Task record with my existing Opportunity record. See below.

Please note:
- This also works by writing it like this opportunities() as below shows, although I find opportunities/ less fiddly.

- And note that I wrote the plural of opportunity so I wrote opportunities. Many an hour was wasted for that little tip.
I see you've used the Common Data Service connector now, but I hope this helps in the future maybe, or if someone else is getting this problem.
Thanks very much,
Garry