Unfortunately FishEye onDemand does not have the text "FishEye" in the footer, therefore the subversion plugin does not detect the URL of a FishEye onDemand instance as valid repository browser URL. Compare with line 150ff in https://github.com/jenkinsci/subversion-plugin/blob/master/src/main/java/hudson/scm/browsers/FishEyeSVN.java. Unfortunately I don't have any specific string you can look for in FishEye onDemand. Maybe you can change the error message to a warning but nevertheless allow the URL.