I had the same issue. It was cause by a server running on the port 80 where Ganglia was launched.
I fixed it by removing the existing server on the port 80.
The ambari log :
java.lang.NumberFormatException: For input string: "<html>"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Long.parseLong(Long.java:441)
at java.lang.Long.parseLong(Long.java:483)
at org.apache.ambari.server.controller.ganglia.GangliaPropertyProvider$RRDRequest.convertToNumber(GangliaPropertyProvider.java:629)
at org.apache.ambari.server.controller.ganglia.GangliaPropertyProvider$RRDRequest.populateResources(GangliaPropertyProvider.java:471)
at org.apache.ambari.server.controller.ganglia.GangliaPropertyProvider.populateResources(GangliaPropertyProvider.java:120)
