The error is coming for me as well. I am using Seven as Administration theme ...

The error is coming for me as well. I am using Seven as Administration theme and while I am changing the themes, I am getting this error.
I am unable to place blocks in regions. I noticed the same error when I am placing the blocks.

In browser console, I can see errors such as anonymous call has been made.

My usage stack is Drupal 8.6.3 (got same error in Drupal 8.6.2, so upgraded the core). I am using Cent OS and in a Virtual Private server.

Any help to on this on where to start looking.