This is where the endpoint is documented [1].
Which version of gerrit are you using?
Have you searched in the error_log, for some additional clue?
Users should be able to star/unstar changes that belong to them, possibly this change does not belong to the caller who is trying to unstar?
You could ask that user to trigger that request manually with enhanced debugging, something like:
```
curl -s -u <username> -X DELETE 'http://<gerrit-url>/a/accounts/self/starred.changes/<projectname>~<changenumber>?trace=debug'
```
Then check in the error_log, to follow what is going on.