Grails: a strange IOException that is rare when trying to run the target run-app

Question

From time to time, I saw this error when issuing a target grails run-app:

| Error Error executing script RunApp: : IOException in C:\Users\user\Dropbox\Code\Projects\qotd\target\work\web.xml.tmp - java.io.FileNotFoundException:C:\Users\user
\Dropbox\Code\Projects\qotd\target\work\web.xml.tmp (Access is denied) (Use --sta
cktrace to see the full trace)

and still it seems that I can always get around it by releasing it again grails run-app. If possible, I would like to know what is the mechanism causing this error, and why it seems to disappear / fix itself. Thank.

Edit: is it possible that this error occurs, because since I save my project in the Dropbox folder, sometimes I can refer to the target grails run-appwhile something in Dropbox is still in sync?