I am having basically the same issue. When the Feral Guardian is slotted as my ultimate (Front or Rear bar, it doesn't matter) I get numbers like 250/0 and inside the little ultimate pic it shows inf%
I can actually summon the Feral Guardian but it usually only sticks around for a few seconds and then disappears.
When I put Sleet Storm and/or Secluded Grove as my ultimates they work just fine.