From the about page:
About This ThreadFix Build
Last commit: 0e15f90
Build date: Aug 31, 2017 08:51 AM EDT
It seems that logging out and logging back in results in the deleted team being removed; however only if there is at least one team that has not been deleted. For instance I had a single team (team A). I tried to delete that team and received the above error. I logged out and back in and team A still existed. I created a new team (B) and again tried to delete team A. I received the same error again. However this time when I logged out and then back in Team A was gone, while Team B remained.