Milestone field allows you to choose the "-------" placeholder (BB-5030)

After the update to the new issue tracker I have problems editing issues.
Procedure (works at least on issue BCLOUD-157 of repository dlopy/mvp):

• open the issue
• click on "Edit Issue"
• change the assignee to something else
• click on button "Update Issue"

At this point, the button turns grey and nothing happens. By bringing up the developer tools of the browser I was able to track the problem down to a "400 Bad Request" communication error with the server (see the attachment).