500 error on sprint board(scrum 0.7.0, redmine 2.3.2)

I used scrum 0.7.0 on redmine 2.3.2 for nearly half a year, these days, there occur such error, it is ubnormal...when we go to sprint board, there is 500 error, but it is not overall issue, I found 1 project with 7 sprints, only sprint 5 has such issue, we cannot see sprint 5's contents, if click to sprint 5, 500 error shows.

The error log is here---Defect #1055

Hope to get help.