Safari extensions have suddenly stopped working (10.13.4) All of a sudden this morning my extensions stopped working. The extensions preference pane is empty and says &quot;No extensions&quot;.
All the extensions are left in ~/Library/Safari/Extensions, and if open one of them to install it says (roughly translated):

Safari can not install this extension
An error occured when the ”NinjaKit” extension was installed.

I tried restoring the entire ~/Library/Safari folder and ~/Library/Preferences/com.apple.Safari.plist from a backup from yesterday when everything was working fine, but that did not solve it.
The issue may stem from wrong permissions somewhere, since if I try to edit the Extensions.plist file manually it says &quot;Unlocking failed for an unknown reason.&quot; when I try to save it. I tried sudo chmod -R 0777 ~/Library/Safari/Extensions but that did not solve the issue either. The file is not locked from the &quot;Get Info&quot; view in Finder.
The permissions issue ended up being the problem that prevented me from using extensions. 

For some reason, it keeps the weird time machine permissions (you can't edit a time machine backup etc.) when I copy over the Safari folder through Finder. It shouldn't do this and never has for me before this.

But I solved it by instead copying it over through the terminal, using cp, which doesn't preserve any metadata unless I specify that it should do that. After doing that, I had to reinstall the extensions (and thus lost some extension data) but at least they work now.

EDIT: I was able to recover the lost data, from a backup of ~/Library/Preferences/com.apple.Safari.Extensions.plist