Email Link Tracking not working.
I'm not sure this is quite the same problem as I had, but it may be worth looking at. I'm in WordPress 4.9, CiviCRM 4.7.22. I was getting Error 500 for all the tracked links and I resolved this by changing [civicrm.root]/extern/url.php to 644 permission and the containing directory to 755. I've had a lot of similar problems (e.g. with getting images into KCEditor with KCFinder. One of the incomplete screens I get there (problem not entirely solved) is the same as I get with the Browse server button that you mention).

I think its all to do with the fact that I have a hosted solution (SiteGround) and it insists on 755 and 644 permissions whereas a lot of WordPress files are 775 and 664, which I think is only appropriate for stand alone hosting (again need more research).