Ugh, I’ve run into that too! Super annoying when a note just won’t save, and the error gives you nothing useful. Quick check: Are you logging the note from the web app, mobile, or through an integration?
Easiest way to test if it's UI-related: open the contact in the HubSpot web app, click "Add Note," type something simple like “test,” and hit Save. If it fails again, grab the exact error (or screenshot), and check the browser console (right-click > Inspect > Console tab) for red flags. One time I found out it was a browser extension messing things up.