Fix “This field was locked by vendor” in Plesk Updates source and installation settings

I was trying to install some new components to one on my Plesk managed servers but I got this error when going to “Server > Tools & Settings > Plesk > Updates and Upgrades”:

Failed to read product information from the file : Can't process products.inf3: Failed to download the package http://20141204-09h35m01s.parallels.mirrors.ovh.net.snapmirror.vps.ovh.net/parallels/products.inf3: The requested URL returned error: 404 Can't process versions.inf3: Failed to download the package http://20141204-09h35m01s.parallels.mirrors.ovh.net.snapmirror.vps.ovh.net/parallels/versions.inf3: The requested URL returned error: 404

I asked OVH technical service and they told me more or less this: “Yep, that URL is no longer available but we only provide it on first automated install. Fix it yourself.”. I have to say that this happened with the customer service from Spain, something that of course didn’t surprise me. We are used to this kind of “help” from customer services here.

So I tried to change the product information source to the official Plesk one but I noticed that the field was disabled, as it said “This field was locked by vendor.”.

I started to search for the original URL in /root/parallels in order to edit it with no luck. When I did a search for this problem on the Internet I got to the most surreal fix I've ever come around: Open the browsers devtools and remove the "disabled" option to the "URL to the directory with .inf3 file" input field, change the URL to the correct one (http://parallels.mirrors.ovh.net/parallels/) and click "Save". It worked!

Ref: http://forum.odin.com/threads/where-is-the-parallels-installer-configuration-file.329704/#post-782253