Error 500 when creating a milestone

Summary

When I try to create a milestone, I get a 500 error. It's an imported repository from github and two milestones imported from it.

Steps to reproduce

No particular step, with or without description, dates, or only with a title, I always get a 500 error.

Example Project

https://gitlab.com/DMaugmentedTeam/DMA

What is the current bug behavior?

500 error instead of milestone creation

What is the expected correct behavior?

A newly created milestone

Output of checks

This bug happens on GitLab.com