Hi Pranay,
thanks for the answer. I really carefully checked all my Subscriptions. But the one mentioned in the Entra Monitoring & health error message is not listed. And as I said, very likely that was one I deleted long time ago.
So I created a new Log Analytics workspace with PowerShell as you described. Btw. thanks for that very good and detailed description. Running the az command to list all workspaces and see it, looking at it in the Azure Portal I have it.
Lastly I checked my Roles on that Log Analytics workspace, inherited I'm a Service Administrator and directly given I'm a Log Analytics Contributor. I waited for some hours and then I checked again in Entra.
But back in the Entra Portal when I click on Log Analytics in Monitoring & health it still shows the error with the missing subscription ID. Entra does not pick up the newly created one.
From my point of view Entra simply picked up something really old and I'm unable to convince Entra to forget all that old stuff and pick up the new ones.
I will open a case with Microsoft.
I would like to say thank you to you, you've been such a valuable helping hand, but I think both of us are at the end of what we can do.