It could be a problem of SPUtility.GetFullUrl not being able to work with layouts URLs (since they are shared between all sites in your farm, and not site specific)
Try if
string fullPath=SPUtility.CombineUrl(SPContext.Current.Site.Url, webUrl)
works better.
Don't really now why you are trying to get a file from the virtual directory layouts though! Check for example here of how to read from this directory: How to read a file from a folder in _Layouts?
If possible, you should consider to deploy the file into a library in SharePoint instead. That way you could do CRUD on it just like any other file in SharePoint