Where does Safari store the homepage URL? For unknown reasons I cannot change the once-set homepage URL in Safari (v10.1 on OS X 10.11.6) any more. If I change it in Safari's Preferences window, under General, it simply resets it to the value it was set to before. Same when I use the "Set to current page" button.

The same problem is with Safari 10.1 on another Mac running 10.10.5. I wonder if it's because they're both using iCloud, sharing their bookmarks. However, the homepage is set to a different page on each Mac, which suggests that this address is not synched with the bookmarks (which is good, IMO). And turning off iCloud Safari sharing doesn't help, either.

So I am trying to figure out where it stores the URL on disk, so that I can check if that is somehow locked.

However, I cannot find the current homepage URL in various places. Neither in the Safari preferences file (~/Library/Preferences/com.apple.Safari.plist) nor in any file inside ~/Library/Safari/ (including Bookmarks.plist) and ~/Library/WebKit/, at least not in plain ASCII text.

Where is that URL stored?
In older Safari versions you could set it via defaults write com.apple.Safari HomePage -string "http://example.com". Safari seems to have done away with that at least since MacOS 10.11 (and possibly earlier). From this device management thread it looks like it's stored in iCloud, but in a rather opaque way.