EDIT (13 July, 06:55 GMT): We have now released 1.13.5 for both Android and iOS which fixes this issue. Please update your game!
There is a known issue with continuing Challenge Event (Guardians of Teltoc) battle with gems. We are currently investigating this issue and hope to get it resolved as soon as possible!
We will release a new update (1.13.5) to fix this issue as soon as possible. In the meantime, we ask you to not use gems to continue the event battles.
Our apologies for any inconvenience this caused! We are also looking into automatically compensating any gems lost due to this issue after the event is over.