Server App is out as free update. This app is compatible for Yosemite (10.10) and El Captain (10.11).
However many user got problem with 5.0.3 and 5.0.4 in Yosemite especially for Web Sites. It seems that Sites are not accessible anymore and This problem seems to be related to wiki service: when the wiki service is turned on, all web sites redirect to wiki.
How To Fix it:
- turn Wiki service OFF or
- upgrade to El Captain and download again the app