Apart from testing the location of the zip file with a browser.
1. It might be (I'm speculating here) that VSK4 is not treated as an browser application. Hence may not be able to read/search (execute) the file due to missing rights or privileges. (I have observed such behavior.)
2. There may be a problem with the character encoding between the Operating System , the Webserver and VSK4. To make sure encoding is not the problem use
ascii AND lowercase characters only. Brackets'[' and underscores '_' all may be cause for trouble.
I suggest you start with the simplest 'aaa.zip' in the root directory of the webserver with all possible rights.
(don't forget to shut down VSK, update the URL in your locater file, delete the yourboatname.zip file in VSK boats skin directory and restart VSK in between. )
If that works then take it from there. (Put it in the right directory, remove all excesive rights, change the name of the file to what you want, or to what works for you.)