bummed for Ben..
https://www.runnersworld.com/news/a20803232/are-sacral-stress-fractures-on-the-rise/
Sacral stress fractures occur more easily in people with bones weakened by osteoporosis, osteopenia, or nutrition problems, including deficiencies in calcium and vitamin D. These conditions are more common among older women, but affect other runners, too—28-year-old Ritchie, for instance, says his bone density scores were low enough to quality him as having osteopenia.
Even runners with normal bone density can disrupt the balance of stress and recovery enough to develop sacral stress fractures. Not eating enough calories in general or protein in particular robs your body of the raw materials needed to build new bone.