After a restore on a testing environment I bounced into the IIS error “Cannot make a cache safe URL for “init.js”, file not found. Please verify that the file exists under the layouts directory.”
You can check if the init.js file is no longer located in the layouts folder, but probably nothing is wrong with it.
It turned out that the Dutch Moss language pack was not installed on the server, which was used by the restored web application.
If you ever bump into this error, make sure you have all the required language packs installed. After you fixed that, the site collection will be fine.